It there any merit to thrash over return? On paper it's Gen V's outrage with nothing immune. Is it because switching in and out with a bug/flying type is an invitation for sneaky pebbles?
If anything were to use Thrash outside of an Inverse Battle, it'd be Pinsir, but I'm not sure it's actually any better of a choice. I haven't found myself often wishing Return his just a bit harder, especially since if it's against a faster foe or one with priority, Quick Attack can finish it off, and Return + Quick Attack does quite a bit more damage than Thrash. Also, I don't often find myself just spamming Return for three turns in a row; it might be different with a Swords Dance set up, but in the battles I've played, the option of changing moves or switching out is really valuable. I'd never use Thrash; attaching such a cost to Pinsir's primary attack is just too risky.
People are going to be running one of these three as power STAB.
Return - 102 BP, no effects
Double Edge - 120 BP, 1/4 recoil
Thrash - 120 BP, locks in for 2-3 turns, confuses at the end of the lock-in
Off the top of my head, Thrash looks like the best option for teams that would use Mega-Pinsir as a cleaner, when resists are out of the way or weakened enough to punch through with Thrash. It does seem like an inferior option to Return though, especially when you consider that Mega-Pinsir only has decent speed and would like to be able to use Quick Attack when it needs it.
When Double-Edge becomes an option via Pokebank, I'm sure some Pinsir will prefer it, especially the ones that expect to use Swords Dance more often. Personally, I've found Pinsir's longevity not a thing to underestimate; it does have 65/120/90 defenses, after all, especially if you can keep rocks off the field. Also, using it alongside Substiture would be a particularly bad idea, and I'm a really big fan of Substitute Pinsir myself. But yes, once it's released, it should be Pinsir's sole notable alternative to Return, and will definitely be worth a mention. Or is worth a mention already, for anyone already playing the Pokebank metagame.
I do want to point out, though, that it has 1/3 recoil; you may be thinking of Head Charge, Bouffalant's personal variation.