I'm late to the party, but it looks like a decision hasn't been made yet. The biggest problem with rain is that there's almost no opportunity cost to using it. Seven turns of Pokemon that already have tremendously boosted stats also having doubled Speed and a boosted STAB is insane. I know we all know this, but actually spelling it out really puts into perspective just how incredible rain is. On top of the incredible power and Speed of Swift Swim users, Politoed itself gets massive boosts to its stats, which makes dealing with rain all that more difficult. It is perfectly reasonable for Politoed itself to outlast the rest of the team, which means there's pretty much a constant source of rain that doesn't require wasting a turn, but merely a slight shift in momentum that is more than recovered when you get to bring out Kabutops, Ludicolo, Omastar, Kingdra, etc. I don't think there's much debate on whether or not rain in its current form is broken, so I'll spare us all some time there and jump straight to where my thoughts lie.
The biggest flaw of rain sweepers is how they can all struggle to break through strong defensive cores. The one I've used the most in Tier Shift to handle rain is Ferrothorn / Gastrodon, which does a good job of handling the main rain sweepers and only really struggles with Focus Blast Ludicolo. Even that, however, can be easily covered with an incredibly powerful wallbreaker, from which there are tons to choose because of the massive amount of viable Pokemon in the metagame. Mega Heracross, Gallade, Kyurem-Black, Moltres, Toxicroak, Sawk, etc. all fit in well on rain because the Speed that causes them to struggle against offense is more than compensated for by the Speed of Swift Swim sweepers. This is where opportunity cost comes in. Between how incredibly easy it is to cover all defensive Pokemon and still trample offense on top of the ease of setting up rain consistently throughout the match, rain is both too easy to use and too good not to use.
Because of this, it's obvious that something needs to be done to nerf rain, and I feel least intrusive way to increase the opportunity cost of rain is to ban Damp Rock. While it will still be trivial to set up rain regularly throughout the match, the momentum shift from the sooner ending of rain makes it easier for offense to respond, and the drop in power and Speed of Swift Swim sweepers makes it easier for defensive teams to recuperate from their onslaught.
Why not ban Drizzle, the combination of Drizzle and Swift Swim, or any specific Swift Swim sweepers instead? While all of these proposals are solutions to the problem, they unnecessarily nerf or completely kill rain as a playstyle while we have a much cleaner solution. Even in its current state, rain is hardly impossible to prepare for or play against, but it's undeniably difficult to do both. Without Damp Rock, I doubt it will suddenly become easy to build and play against rain, but it's much more possible in much more ways, while rain itself will still be playable. If we take this route and I turn out to be wrong, no real harm is done. We're already over half way through OMPL, and there are no other major Tier Shift tournaments. What I'm saying is that we have time to go further in the event that I'm wrong and rain is still broken without Damp Rock. Even so, OMPL would be the perfect testing grounds for the new metagame if anyone feels frisky enough to try post-ban rain in a tournament setting.
As a final small aside, I could see rain without Drizzle or the Drizzle Swift Swim combination still being broken. Uxie, Klefki, Liepard, Volbeat, etc. are all fantastic potential rain setters, and many of them have access to U-turn to pivot into a rain sweeper as well. Politoed is great because you don't have to spend a turn using a move in order to set up rain, but seven turns of rain is still seven turns of way no matter how you spin it.
