When looking at army list advice online, I commonly see Space Marine players discouraged from taking any assault units. The line of thought is that Space Marines are a shooting army, and any points spent on assault units are better spent on more firepower. This is compounded by the generally low opinion of Space Marine assault units. They generally either don't hit hard enough (Assault squads) or are too slow without an expensive Land Raider transport (Assault Terminators, Assault Centurions, Honor Guard).
This is true up to a point. An assault oriented Space Marine army is going to have a hard time on the table. However, an army with no assault units is going to be lacking some options that leave it vulnerable in certain match-ups. Space Marine assault units aren't going to tear up the table like Thunderwolves or Flesh Hounds, but they serve two important functions: they wipe out enemy units that are resistant to shooting and they tie up dangerous enemy units, effectively removing them from the battle until assault is resolved.
The first task of an assault unit is to deal with enemy units that are tough against shooting but weak against assault. This often includes shooty units with good cover saves, such as Space Marine Scouts, Eldar Rangers, and any Nurgle Daemons. However, it also includes more resilient units that are good at shooting but lack close combat punch, like Space Marine Devastators or Tau Broadsides. While your assault units may struggle to wipe out these units, you can easily keep them in combat for most of the game and prevent them from shooting.
For this role, the most important aspect of your assault unit is speed. The unit doesn't need to hit hard as long as it can engage the enemy. The unit should also be relatively cheap; if they need to tie up an enemy unit for multiple turns, you don't want to be losing that many points. I find both Assault Marines and Bike squads are great at this role. Conveniently, both of these units fill a required slot in a Demi-Company, so you have a good reason to take them rather than the cheaper Land Speeder or Attack Bike.
The second main role of an assault unit is to tie up an enemy death star. Quite often, death star units will be stupidly hard to kill with shooting. High toughness, invulnerable saves, or Invisibility combine to prevent you from stopping them from wiping out your army, no matter how good your shooting is. However, you can tie these very expensive units up in combat (as long as they don't have Hit and Run, which seems to have become much rarer than it used to be). For this role, durability is more important than anything. For Marines, this means only units armed with storm shields need apply. This limits your choices to Assault Terminators, Vanguard Veterans, and Command Squads.
I discovered just how vital this role is when using my bike Command squad. It's a pretty decent assault unit; it won't kill a true death star but it will mulch through just about anything else. I was amazed how many players just didn't have an answer to such a unit. Without any assault unit to answer my Command squad, it could wipe out multiple shooty units each turn with a multi-charge while soaking up any return fire.
What's you experience? Do you think Space Marines need some sort of assault threat or do you have success running a purely shooty army?