You forget that flash cannon is special? The one poke that can actually utilize it well is magnezone, who often sees ttar switch into it. Even special gimmicky metagrosses prolly have better options.Aura Sphere is a good enough STAB attack, but Flash Cannon doesn't have as much power as Shadow Ball, Dark Pulse, or even Dragon Pulse. It's resisted by more, and hits less for SE damage. I consider it an inferior attacking type, and is only useful on Scizor and Metagross, or the occasional Probopass.
EDIT: In conclusion its only good for pokemon with poor special movepools and stab :P