I never see these in any army lists. Do they really suck, or are skinks/saurus just that much better, or something?
Consider this; rather then 5 wounds of swarm you could be getting 4 wounds of Saurus. You lose unbreakable and some speed. However, you gain alot of survivability, the ability to kill things and lose crumbling.
Basically yes, they do suck. Since they now crumble (they didn't in 6th edition) they are underpowered and only a few people have niche uses for swarms in any army. GW destroyed them because they used to be too good so they got nerfed too far.
Well I used them a while a ago and i kinda liked them.. but like Krox said.. you could get better. I used 3 of them.. planning was to get into someone's rear and throw 15 poisoned dices. It ended up being 10 dices killed 2 Wight Knights (Those cheap OP VC Knights) with it. In the end all 3 of them died but it took him a while to do so.. Not saying they are totally suckage.. but there is most of the time something more useful to be taken.