The point of higher CP (besides getting higher placement in gyms) is getting off your special attack in time and make them waste potions and revives. Like, sure Tyranitar and Rhydon get drown by Vaporeon, but if they have enough bulk they can throw a Stone Edge before they faint. Not everyone lives in dense pokestop locations, so the less pokestops around the more effective high cp pokemon in high lvl gyms are. I'm pretty sure your Jynx would still have trouble fighting 3400+ cp dragonites with dragon claw, especially in multiples.I've been playing since launch and only have 2 Dragonites over 3000 CP.
There's really no reason to go that high though, unless you are looking to get a top slot on really competitive, established towers. In my experience, you really just need your attackers around 2300-2500 CP, and your defenders at 2500-2750 CP. Type advantage is such a huge edge that CP isn't all that important, once you are in the endgame. A 3200 Tyranitar will get curb-stomped by a Vaporeon regardless of whether it is 2200 or 2800. My 2100 Jynx absolutely wrecks Dragonites that are above 3000.
And yeah, type advantage isn't that huge. My 3000+ Vaporeon can't faint a 3000+ Rhydon before it stone edges and Rhydon has a double weakness.