Boomkins are not taken for damage because of what a lot of other people have said. Boomkins are taken primarily because they can help land kills on targets. The ability to cyclone, root/beam, and stun all at once means they can solo CC three healers. Coupled with the ability to put out three starsurges into kill targets it is a lot of damage. Good boomkins abuse the pvp set bonus to keep starfall up nonstop but still bank 3 charges of starsurge for kill attempts. Boomkins are also terrible targets because they have a passive ability that makes their damage instant cast if they are being hit. I have literally soloed people in group fights because a rogue was sitting on me and all I did was shit out starfire and starsurge instant casts as quick as I could hit the button.

Boomkins also are pretty decent flag spinners, and in a pinch they can run flags if needed. They also have the ability to put out a little healing to ease aoe damage intake or cyclone those terrible DKs off their healers.

Boomkins just have a great toolset for RBG. About the only thing they lack is some sort of dispel punishment, but that would be a little too strong.