Ok relax man...
I know my memory of exactly what spell does what is a bit foggy.
(Could have sworn that was Toughnes....

)
Yea if you are playing against magic heavy Deamons of Chaos,
you probaly are going to loose the game if you are taking any fluffy units.
this goes for any army, not just us.
Yea we all know swarms probaly suck, the point is
are they any use at all?
you are never going to want to get them charged,
Altough they probaly have better initive than Saurus,
but then again what Doesn't ...
The best thing I can think to do with then is charge a
block of something in the flank with them,
only one rank will be able to hit you and hopefull with only one attack.
so swarms have what 3 or 5 wounds...?
you should have a wound left so you can strike back with.
Stats sugest that they would be more effective than 2 ranks of Cohorts
in a similar situation, as far as attacks go...
Even then thy would die to crumbeling... (Darn you 8th edition)
Like a lot of 8th edition you have to use combined charges,
if you have a unit engages with saurus in the front and swarms on a flank,
I could see them as posibly being effective, since they will probaly
only have 3 or 4 attacks comin their way and the saurus block
should keep them from crumbleing to combat res.
The best advantage I can see them having is their small size...
with all the true like of sight rules they can hide behind the tinyest of terrain,
also being skirmishers attacking a unit in a forest will be to their advantage,
perhaps they would be good at attacking skirmishers then...?