You may not need Farseers to unlock the Seer Council any more, but they bring some psychic powers to the unit that combine incredibly well with the Warlock's powers and traits. For the boosts they offer to defense, offense, versatility I think they're just too good not to take if you want a proper deathstar. The thing to look at when thinking about Farseers is the powers you are aiming to get and which of the three charts to spend your rolls on. Your aims may be influenced by your list, Warlock powers and Remnants of Glory. The first set of powers I'm going to go over is:
Runes of Fate
Guide
The usefulness of Guide in a Seer Council will depend on a number of factors. For the Council itself, it will matter how many Singing Spears you brought and whether or not you even have any need to throw them. And it could affect any witchfire powers you're going to throw out, but the ShuCats are already twin-linked and it will be wasted on them. Otherwise it will matter what the rest of your list looks like and what shooting units are going to be within 24" of the Council. Obviously this excludes any Wave Serpents already twin-linking through TL Scatter Lasers, but it could be a nice boost for Dark Eldar allies who are keeping close by to assist the Council with Grisly Trophies. If I planned to have someone useful nearby, I wouldn't mind trading for Guide if I get something I don't at all want. Maybe even one on each Farseer if I have a lot of supporting fast firepower and am just not getting the rolls I want.
Executioner
I'm not at all a big fan of witchfire powers. The few that are better than guns still need a Ld test, can still be blocked by Deny the Witch, risk Perils of the Warp, cost valuable Warp Charges, use up an even more valuable roll on the chart AND need to roll to hit. If you could trade the roll for a good gun, I gladly would. That said, Farseers don't come stock with good guns. You could buy the relic long rifle, but it's expensive and unnecessary to the goals of the Council. If I already have Guide and can't swap, Executioner could be of value. At the very least, it is better than a TL Shuriken Catapult. You could use Executioner if you absolutely have to and since I want us to be able to do something with each roll we make I have to look at this power's potential to kill off someone you'd like dead in a squad. And its ability to wound high toughness targets. But you can only cast a single Witchfire power per turn and while I'll go over this in great detail later, I'd much rather have Psychic Shriek.
Doom
This power is better than it's ever been due to its new utility against vehicular targets. The Council already wounds everything on 2s and has Armorbane against vehicles. Doom helps the Council most when firing ShuCats and when swinging witchblades against AV13-14 targets. Still, it could be a huge boost for Dark Eldar allies with poisoned shots and it's great for bolstering the effectiveness of S6 dakka vs vehicles, so I'd be thrilled to get it. If you roll Doom and Fortune, your spell casting is pretty much decided each turn, but what if you wanted to stack your odds of being able to use all three powers each turn? The first thing that comes to mind is to take the Spirit Stone of Anath'lan on a Farseer; it lets you trade your invul save to reduce the warp charge cost of a power. With shrouding, it is likely you won't be rolling your invul anyway and you can play it by ear if you run up against AP2/3 shooting with ignores cover. This would mean that at the list-building stage you already know which Farseer you are rolling more than once on the Runes of Fate with. That reduces flexibility a little, but the pay off would be big if you rolled both Doom and Fortune.
Eldritch Storm
Here's another witchfire power that I'm not likely to want. It is both better and worse than it used to be, but regarless it doesn't compare well against many of the blessings and maledictions available to us. I wish it still rotated tanks. At least with fleshbane it might wound tons of stuff if you get a good shot off against some bunched up targets, but who doesn't spread out when they see you roll up large blasts? It also has haywire rules now and can damage tanks, but so can a S9 Singing Spear. And with haywire, ES is likely to just burn off a single hull point from the target. This is another one you might be able to use, but since it competes with Psychic Shriek and costs 2 warp charges it could just get little to no use when you need to be casting more important powers.
Death Mission
I see a lot of potential in a Farseer casting Death Mission and riding with his Warlock posse. The problem is he needs a better weapon to really take advantage of the buff. When using 2 Farseers, you don't know which Farseer might get this so each one needs a good relic weapon. That bumps their cost up 15-30 each, depending on what you buy. With a Jinxlock I think the Firesabre would be great since the AP3 would be just as good as AP2. The Soulshrive from the Iyanden supplement could be nasty too. And with WS10 and I10 a DMSeer would rip people apart in challenges when using the Shard of Anaris (as a bonus it also gives the user, and by extension the unit, Fearless). Fleshbane, Rending and Instant Death in challenges is no joke with a Death Mission stat line. Basically if you manage to get this power on one Farseer and Fortune on the other I think it might be worth using it if you want a beatstick to stand in the stead of an Autarch. You wouldn't be able to cast the other excellent powers with him afterward, but with the right equipment this can make a Farseer an absolute monster.
Fortune
Obviously this is the money maker. It combos incredibly well with Conceal and Protect, providing durability that the old Council could only have dreamed of. It also picked up rerolls on Deny the Witch, a valuable and unexpected improvement. Once you have this you can relax and roll for stuff you feel like using, in my opinion, as this is almost the whole reason I'll be rolling on the Runes of Fate. While Death Mission would be fun and probably good if I have the builds for it, and while Doom could help support the rest of the force, none of the other powers make or break the Council. Despite Fortune's importance, I think there is less to say about it because everyone knows how amazing it is and how much the Council needs it. I'll just say that rolling on the Runes of Fate multiple times to get Fortune is risky as not all the powers are great when compared to some of the things we want to do with Divination and Telepathy. Just be careful with how many times you're willing to roll because you could get yourself stuck in a situation where you don't roll anything you need and have no more rolls to spend on other charts. That goes against the versatility available to us in rolling for many of the best psychic powers in the game.
Mind War
And finally the last result on the chart, a focused witchfire power. Mind War now works a little differently. You're not guaranteed to kill the guy you want and if you roll poorly you could debuff yourself. Not a problem, really, since Farseers are not the only thing we're relying on to do damage. It does work well with Horrify since debuffing the enemy Ld could help you kill them. And at least these days it ignores cover, but so does Psychic Shriek which can kill way more guys and combos just as well. I don't really want this power, but it certainly has some worthwhile use. The warp charge cost is high, so it could potentially fall to the wayside if you need to cast Fortune, but I'd put some thought into whether or not I'd swap for Guide because depending on what powers other characters have roll this might be of value.
In extreme summary: Fortune.
In a regular summary: take care that you don't get too greedy when rolling to get Fortune. The other powers have their uses, but with many of them costing 2 charges you may not be able to cast enough powers and your Farseers won't be operating at their full potential. Even though we can technically put many of the weaker powers to some use, charts like Telepathy and Divination have much better powers overall and rolling on those charts would be much more beneficial than getting stuck with a lot of Runes of Fate powers and no Fortune. User beware.