@Wyddr: 2.22 kills on average, that usually are the 2 flamers, problem is these are wounds taken in your turn, when you should be delivering wounds, not receiving them.
rant:
[Spoiler]
However on the charge the stormies should be at their best since they are supposed to be a CC unit, in a unit of 10 stormies there will only be 8 left that can produce 1.33 wounds on the charge.
Even against a combat squad this would be bad, and I haven't even taken the return attacks of the marines into account, 9 marines statistically kill 2 guardians.
Of course the stormies should be shooting on their way into CC, with 2 flamers hitting 3 models each that's another 1.5 wounds(including pistols and their pseudo-rending).
Against 10 marines the result would be 1.5 marines killed from 9 marines shooting,and 2 guardians killed from overwatch, then 1.33 more marines killed in CC and 7 marines strikes back for 1.5 more dead guardians.
To sum that up that's 3 dead marines and 3.5 dead guardians at optimal conditions for the guardians, during the eldar player's turn.
Against a combat squad it's still 1.5 wounds from shooting so only 4 marines overwatching, that kills 0.89 guardians. In CC the guardians will kill basically the same 1.44 kills, so 2 marines swinging back for 0.44 wounds.
Sure the guardians wins by 3-1 in their turn, but with just a little deviation from the statistics the marines still will prevail in the end despite optimal conditions for the stormies.
A couple of pot shots from outside rapid fire range could the turn before could easily tilt this in the marines favor.
The argument "but the stormies will be in a serpent the turn before" well how then are they able to assault at all?
[/Spoiler]
end of rant.
The use of stormies is of course to go after your opponents specialized units, such as assault terminators or scouts. Against most troop choices stormies will suffer, IG, marines of all flavours, tau, DE, they will even struggle against orks, crons and nids.
The stormies will even be outnumbered point for point by everything except marines and crons.