I use it alongside my bite/crunch gyarados against lugia. Bite is super effective into lugia and as you said it has the best hydro pump so it does good damage all while resisting hydro pump from Lugia. Plus my bite/hydro is like level 35 or something so it's already high enough to stay in the fight for a bit. That's the only case I can really bring that's from my experience. It's really helpful, I enjoy using it, he's one of my favorite mons to use but I don't use bite/hydro pump anywhere else really.
Is there any good use for a Bite/Hydro Pump Gyarados?
I have a bunch of TMs saved up (17 fast, 19 charge), and that's after having used a few on my more obvious candidates. I put a lot of dust/candy into powering up a trio of Gyaradoses back when CP mattered, but now it doesn't so much. I TMed one to get DT/Outrage, which I figure will be useful if Dragonite raids ever are a thing. I TMed another to get Bite/Crunch, which is especially useful against a Hydro Pump Lugia, should be also useful against a Focus Blast Mewtwo, and should be a solid psychic counter in general.
My third has Bite/Hydro Pump, which I've kept for now as it's near-optimal for DPS, but I can't really figure out what I'd use it against that it's optimized for. It's got the best Hydro Pump in the game right now, but unless they ever change its moveset to include a water fast attack, I have a bunch of Vaporeons that seem like they'd be better choices in battle. I'm considering using a charge TM, and keeping it Bite/Crunch if it gets Crunch, or if it gets Outrage, using a fast TM to get another dragon combo.
I'm not sure I want to waste the TMs on this yet, but I'm just curious if anyone can make a solid case to keep a Bite/Hydro Pump Gyarados.
Answers
Yeah, I use it on Lugia raids as well, I'm just trying to figure out if it needs Hydro for any reason. If I'm just using it on Lugia, then it'd be better off with Crunch (and then more useful against Mew/Mewtwo eventually). I'm not sure I'd burn a charge TM when it might roll me an even more oddly suited Bite/Outrage combo, but am considering it if it might make that Gyarados more functional.