So I wanted to wait to check my book before commenting on this thread again, since one part of the GW website says swarms have M4 and another part says M5. And it is on exactly the same page describing jungle swarms.
Movement 5, fairly decent given you get 5 poisoned attacks from each base. Yes you have had pretty decent luck so far, Itzi-Bitzi, and it will take more than a few games of luck to even make most people here slightly reconsider. They are very expensive points wise, you cannot deny that. If they were 30-35 that would be more fair.
Lets see what might happen if you grab a unit of 3 though. Since they are only s2, I will ignore normal hits and wounds, they will on average get 2.5 poisoned wounds (3 bases). Not really as much as I was expecting when I started typing before I did the calculation... Better add in the normal wounds; maybe an extra 1-2 wounds against basic infantry for the hits that aren't poison.
For your trouble, you won't flee but will crumble. You now need to lose combat by less than 5 if you want to keep the unit completely effective in the next turn. You will be facing 3 ranks, standard, outnumber. 5. You need to outkill whatever unit you charge, if that is indeed your tactic. Lets be optomistic and say you get 2 wounds total (assuming your opponent is the standard t3 infantry with 4+ save*, half the poisoned wounds are saved and only .83 of the other hits gets through. Ouch) this leaves a unit champ and 2 others, so 4 attacks back. 2 hit, 1.3 wound, you are looking at 1-2 wounds back. Sometimes a lot more, sometimes a lot less, all we can do is look at average which is afterall most common/likely outcome. Chances are, you will not outkill your opponent, and this is with 3 swarms remember.
I was actually going to advocate swarms at this point, but looking at this... It is hard. I was going to suggest combining them with a skink (krox?) unit, where the swarms charge the front of a unit while the skinks line up on the flank ready for next turn. Maybe I have just come up with a poor use for them, but I guess even if a base dies and leaves only two when the skinks charge you are still getting rid of the 3 ranks, adding a flank charge to negate outnumber and maybe even bringing a banner to swing the combat. The 2 swarms need to survive the opponents turn first though.
I think I'm too out of it to finish the analysis. Poor use for swarms on average, though there will certainly be lucky cases like Itzi had. If I was going to double team, for the points of 3 swarms I'd rather have a small unit of saurus.. This leads us back to their potential use as units of 1 for small speed bumps or to direct charges. And that has been discussed already.
Well, I certainly tried but unfortunately I'm still not convinced, and I'm not sure how you have spare points to throw around on them. The LM army is expensive, I always find myself struggling for points and trying to squeeze a few things out to keep within the points. Maybe in an all skink army they would be easier to justify, but when you grab a lord choice, a steg maybe, and some saurus you are definitely on the struggling side of points rather than 'hmm what should I throw this extra 100 points into?'
*Of course swarm friends are going to say they are better against high toughness low armour troops. Generally, these kind of troops are better than just s3. Ghouls being the most common example these days IMO, you will kill an extra 1 or 2 but in return they have 2 attacks each anyway so you gain little overall in my example
