Damp rock banned.
Uxie (say) is in, sets up Rain Dance (1st turn of rain).
Uxie U-turns out to Kabutops (2nd turn of rain).
Kabutops Swords Dances (3rd turn of rain).
Defender switches in Water resist to sponge Waterfall (4th turn of rain).
Defender sacrifices a Pokemon to Kabutops (last turn of rain).
Now Kabutops is entirely vulnerable. It's got no rain, no speed, and it's at risk of dying to whatever switches in next. +2 Adamant max Attack Life Orb Aqua Jet will hardly scratch Venusaur for example. It's gotten a kill, but Pokemon faint in Pokemon battles and there's nothing special about this one. Now it's forced to switch out. On the other hand, if rain were still up then Kabutops would be free to use the much more powerful Waterfall (rain boosted too) to force his way past Venusaur. You have to survive Kabutops for much longer with Damp Rock than without.
It's not completely vulnerable. If this became the scenario, people can easily elect to run max speed and Jolly on Kabutops, so at worst, can speed-tie with base 80s and still OHKO them without the Rain. Let's look at the Pokemon that can speed-tie or outspeed Kabutops out of rain (Just UU), bold are ones Kabutops can normally beat:
All calculations are done for a Jolly 252 Atk Kabutops:
*Altaria (Can't really threaten Kabutops much anyway)
*Blaziken (Must win speed-tie for Vacuum Wave)
*Drifblim
*Kabutops itself
*Mesprit
*Venusaur
Milotic (Doesn't usually run Max speed anyway)
Toxicroak
Qwilfish
Nidoking
^Cresselia
Hitmonlee (Jolly +2 LO Aqua Jet still OHKOs 40/0 variants, although Hitmonlee can Mach Punch, will not kill if Kabutops is in good health)
^Porygon-Z
^Moltres
Rotom
Uxie (Doesn't usually run max speed)
Leafeon
Houndoom
Drapion
Arcanine (Aqua Jet OHKOs Morning Glory Arcanine after Stealth Rock, even after Intimidate)
Scyther (Aqua Jet OHKOs after Stealth Rock, but that's kinda whatever, as Scyther takes 50% from it anyway)
Mismagius (Aqua Jet OHKOs sweeper sets after Stealth Rock)
Froslass (Can kill with +2 Stone Edge, but you'll probably end up Destiny Bonded)
^Raikou
Ambipom (Has slight chance to OHKO after Stealth Rock, and will definitely OHKO after SR and Spikes, however, that's probably a bit too much. Also gets Faked Out first turn, which Kabutops resists)
Sceptile
Dugtrio
Alakazam
Swellow
^ Denotes a suspect
* Denotes Pokemon Kabutops speed-ties with
So, as you see, Kabutops can rape quite a lot of Pokemon faster than it even without Rain, and this is not to mention those poor fools that he outspeeds. He is still sweeping a big portion of the metagame, even without Rain. So, is Kabutops broken?? Possibly, and Rain does push him over the edge, yeah.