Yeah, I agree that Thrash under normal circumstances is a bad option, however for late game play, or with sticky web support, it has the potential to blast through walls that might otherwise end your sweep. +2 thrash can straight up OHKO Gliscor who might otherwise be relied on to ice fang Pinsir, which can end his run if Rocks are in play, or he has taken a priority hit. Obviously don't do this if Talonflame is on the scene, but with Jolly you will be outspeeding anything that doesn't have speed investment (ie. Most bulky mons) and anything below 105 base speed. It also presents more of a threat to Skarmory, able to 2HKO with rocks and hopefully end its rampage and switch. (this is from experience, not calculations, so I could be wrong.) In addition, those 18 points multiply out considerably when factoring in Aerialate and STAB, plus either a Moxie boost or SD. It's not downright better, but it's another option, particularly as a last ditch "screw you".
at least it's better than Giga Impact?
at least it's better than Giga Impact?