Ho,

Fly back to do the impossible (that might be a little dramatic... how about difficult?): Make a case for the much maligned heavy bolter.
And make a case I will.

I won't do this like a "pro's and con's" article, because I feel like there are all sorts of obvious cons. Those cons you might have read on forums, other blogs, from actual games, etc. I also won't languish in the past, in the glory days when the heavy bolter could kill Marines reliably. No, the past is dead to me.

I'm going to handle this sort of like a court case. I'm defending the nearly indefensible. I'm trying to get the obvious mass murderer acquitted... or at least not executed. I want to make the case that the Heavy Bolter is worth it. Its worth it in your army lists, its something that can be used to help you win games. It can snatch victory from the jaws of defeat. It can heal the sick, save your marriage and get you promoted at work. If Jesus needed a weapon in the Warhammer universe, it would undoubtedly be the HEAVY BOLTER.
Whoa, whoa, whoa, let's not get silly.
The Heavy Bolter is a heavy weapon that can be taken by most Imperial armies and Chaos Space Marines. It has a stat line that would suggest its battle field role: anti-infantry heavy support.
The actual stats are:
Heavy Bolter Range Strength AP Type
  36" 5 4 Heavy 3

Range.
36" outranges all regular infantry weapons, with only the Tau pulse rifle coming close at 30". This is important because in a vacuum you can outshoot your normally intended targets without taking any return fire. That seems important.
36" also allows you to reach the back of the board in Dawn of War deployment if you deploy up as far as possible. A 36" range covers most of the standard 4' by 6' board if you can get central.

Strength.
Strength 5 is an under represented strength value in 40k weaponry. Prior to Tau dominance, strength 5 was occasionally represented by Tesla Immortals, Psybolts, Heavy Bolters when they couldn't be traded out and furious charging Death Company. Now with Pulse Rifles falling out of the sky, strength 5 is seen more often, but not nearly as much as strength 4.
Str 5 allows you to reliably inflict wounds on infantry models. The 5th Edition paradigm was S4 vs T4. That's a 50% wound output. Hitting with 10 bolters against other Marines, you're wounding 5 times. S5 vs T4 means you wound 66.6% of the time. That's an improvement if you can somehow manage a similar number of shots.
But wounding T4 models isn't what I want to focus on.
What do Eldar Guardians, Guardsmen, Cultists, Fire Warriors, Pathfinders, Acolytes, Pink Horrors, and Daemonettes all have in common? Toughness 3. Each Heavy Bolter shell will wound 83% of the time. You still gain the same amount of 'goodness' when upgrading 4 to 5 against T3 and T4, but we'll see why that matters shortly.

Armor Penetration.
That was short! AP 4 in a past edition might as well have been AP -. If it wasn't AP3 or lower, it was pointless and never came into play.
We live in 6th edition now. And in 6th edition, power armor is outnumbered. Even with the new Marines running around, large and highly competitive GTs are still seeing Eldar, Tau-Eldar, Eldar-Tau, Eldar-Dark Eldar, Tau filling the top spots over and over again, with Necrons and Daemons sneaking in here and there.
Suddenly, AP4 seems a bit more important. Ignoring the armor save on that list of T3 models is... AWESOME! And we can even add in the Necron Warrior into that list, being a 4+ armored model.
S5 AP4 will be ripping through Fire Warriors and Necron Warriors alike, as if they are wearing t-shirts instead of, you know, armor.
10 bolters puts out 10 shots at 24". Of those 10 shots, you score 6.666 hits, and 4.4444 wounds. You kill 2.2222 Fire Warriors after armor saves.
The squad next to your Tactical Squad, your Devestators, also at 24" fire their 4 heavy bolters. 12 shots, you score 8 hits,  6.6666 wounds and 6.66666 dead Fire Warriors. Even in cover, the Heavy Bolter squad still outkills the Tactical Squad.

Type.
Heavy 3. The hardest thing to justify. Three shots is a high rate of fire. Heavy limits you to standing still and shooting or moving and snap shooting.
But lets look at the silver lining. The relatively high rate of fire per gun makes it much more effective at snap-shooting than other heavy weapons. Where does this come in?
A game is on the line! It's the top of turn 6 and your opponent attempted to win the game at the end of turn 5 by dropping off all of his 5-man Necron Warrior squads on three objectives! As your turn progresses, you're coming down to the last objective. You've calculated that during your shooting phase, you're going to need to pour a whole lot of fire at this last 5 Warriors squad. You get everyone into position and open up. The last squad to fire is your Devestators, who are snap shooting because they moved. There is only ONE warrior left.
You fire 12 shots and hit twice! You wound 1.3333 times (on average!)! That's a dead Warrior. Four lascannon Devestators don't even average a hit.
There is an advantage to having a higher rate of fire.

But there is a cost. We know Freedom costs a buck-0-five, but what does the Heavy Bolter cost? Depending on your book, somewhere around 10 points plus whatever you need to trade in to get it. In some cases, you trade nothing. In others, you trade a bolter. In others, you pay a little less because its on a "worse" model, like a Scout. In other cases, they are inexplicably less, like on the Vendetta (10 for 2 with no trade ins! ON A VENDETTA!) Either way, you can safely say a Heavy Bolter costs 10 points.

Lets try to look at them in a vacuum. Lets pay the 10 points each and tack them onto a Space Marines Predator. You bump up a 75 pt tank to a 95 pt tank with that upgrade. For 20 pts you increase from two S7 shots to two S7 shots and six S5 shots. Those 20 points are critical in turning a Predator from a 75pt pile of poop into a 95pt anti-infantry killing machine.
The 10pts on the Vendetta is even crazier, turning your tank hunting flying transport, into a tank and infantry hunting flying transport.

So where does the Heavy Bolter really excel in my eyes? On the 6th edition battlefield of the far future, games are won and lost by controlling little points on the table top with infantry. Infantry comes in all shapes and sizes, but commonly in small units of weak guys. The Heavy Bolter shreds those units so your little infantry models can hold the point instead.

Things get a little more interesting in areas where you can make the generic Heavy Bolter better. Grey Knights can mount them on vehicles and purchase PsyBolts, turning them into S6 AP4 HVY3 mounted on 5 man transports. A sometimes important upgrade, letting them threaten light vehicles more reliably, double out toughness 3 models and put out more wounds on monstrous creatures. Sisters can get a faith upgrade (which is much worse in its current incarnation) that gives you rending for a turn. And, my favorite, Imperial Fists, hit more often with theirs and can threaten light vehicles with Tank Hunters on their Devestators.

So, as I prepare to rest my case, I have a request for you. For all of you Imperial and Chaos players out there, try out some Heavy Bolters. Try em out. Dust off those unused Devestators or Havocs. Pull that dakka-Predator off of the shelf. See where you can fit in a few Speeders. Try them and see if they can pull their own weight now. I thought about this the last few games I played and often thought "A Heavy Bolter here wouldn't be bad here" as I was shooting into Cultists on an objective. I imagine you won't be disappointed.

*the defense rests*

*the prosecution stands and says:*

"Cover. I rest my case."

... and that day, the Heavy Bolter was sentenced to another edition of Warhammer in hiding.
Till next time!