The 5% crit from a boomkin doesn't stack with elemental focus so i wouldn't focus on that specifically..
But like you said, you could use a druid to tank when ur going to do heroics and such and go boomkin when ur going to pvp.

I play with a paladin as a tank and i'm loving it, but that's just my opinion
But if you eventually wanna do heroics, i would go with a class that's capable of tanking (paladin, druid, warrior, dk)
I know ppl are able to do heroics with 5 shamans but that does need good gear and such..

Hope this helped a bit!