Ok, I plan on throwing a swarm or two at my players soon, but from what I see in the d20srd about Swarms they don't seem to entangle or snare or otherwise keep people from just moving away. It doesn't even seem to count as difficult terrain.
Is that planned? It just doesn't seem like it makes much sense to me. If someone is covered by 5000 tiny spiders and they all bite him, he can't just take a 5 foot step and suddenly they're all gone. They don't even get an AoO because a swarm never threatens.
Heck, the way I see it, I could on my turn walk straight through a swarm from one side to the other, and could do so without problem.
Is that really how it works? Seems kinda lame.
Yeah, you can do that.
In exchange, the Swarm doesn't need to roll any attack rolls (they move over you, and you get hurt, period), and many can only be hurt by splash damage/AoE effects, so your big dumb Fighter/Barbarian, all your archers, really anybody except your casters, an Alchemist, or people who stock up on Alchemist's Fire are boned.
So it's a good thing they can just walk through them and run away. If swarms entangled they'd probably be the biggest party killer in the game at low levels, and not much better at higher ones (I particularly hate the ones that deal ability damage).
Yes. That's intentional. Swarms are ridiculously difficult to defeat unless you prepare for them ahead of time. Admittedly, most properly paranoid PCs do keep a few ways on hand, but if you don't for some reason, you're hosed. Being able to run away if terrain permits is entirely and wholly intended.
Of course, if you feel that's too easy, design the terrain so they can't just run away.