How useful is Rayquaza generally?

Considering how rarely I use my lvl 30+ Dnites (except as fill-outs in raids where I lack good primary pokemons) I think that 0-1 lvl 30 Rays would be sufficient. Ttar B/SE (Blissey) and Raikou TS/WC on the other hand, I use them all the time...

You said it yourself, it's a better Dragonite but not that much better. Now, against Latis, Dragonite and Rayquaza are by far the best so if you want to prioritize those raids, 6 good dragons would be advisable. Note that the DPS difference is enough for Rayquaza to frequently get close to or even the same TDO as Dragonite, which is to say it doesn't suffer much from the lower tankiness.

Don't need Dragonites with outrage after him. All of my recent Dragonites have Draco Meteor and i'm keeping them that way for a reason.