Yeah Starfire is mostly used for Bosses, but honestly when I am running instances as a Boomkin tank I have found that using Starfire to open by far generates the aggro I need. Not to mention the chance for Celestial Focus proccing. Yeah Wrath is our spam button, but not nearly as efficient as starfire. When spamming Wrath I throw in Starfire anytime Nature's Grace procs.

Standard rotation for me is:

Starfire (Opener) or Hurricane if multiple mobs
Moonfire / and sometimes Insect Swarm (low mana use for swarm)
Wrath
Wrath (usually get Nature's Grace to proc on second or third Wrath.
Starfire (if NG Procs) else refresh Moonfire
Continue to wrath or SF if NG Procs.