If weather is a prominent as you say, then why wouldn't an experienced battler just switch in their own weather and put a cap on Excadrill? Secondly, Excadrill may be fast, but any fighting type with priority (or something with aqua jet) is going to completely screw Drill over. More often than not they 2HKO him. (Even though anyone with half a brain would switch out)
As for things like Kingdra, you're forgetting something. Kingdra has more or less always been good and Swift Swim just gave him a little boost. The only really useless pokemon that becomes a monster with weather that I can think of is Quilfish. His attack matches that of Kingdra, and with Swift Swim he's able to speed-tie if I'm correct. Otherwise, I'd have to say that weather just takes already usable pokemon, and makes them even more usable.
(Also, not ever sand team has those ;; Look at mine)