Stormlash helps to prevent those 1% wipes. If you didn't use Stormlash, and you have a 1% wipe, you know that you could have prevented that wipe.
Its also grand for burst fights, like WL heroic.
In a 25man there may be a balance, but in the end stormlash is not mandatory because the raidwide damage could be chalked up to the fact neither spec is top-of-the-pack material. A 1% (and possible sub 1% when not on fights like elegon) raidwide damage increase is much weaker than many things.
Your argument of stormlash effectively means it would've been impossible to raid without the 3% raid damage buff in cata. Plenty of people did it. It was nice to have but not the end of the world if you didn't.
ya a simple raid CD mate
Stormlash itself may not grant you boss kill, but if you have few exceptional players, perhaps then they can make up the gap for one bad dps, dead player etc.
Ye you are right, but you can say this about any spell or ability in game tho if wont use all of them, you wont kill the boss.