I think the key difference between perma-Rain and perma-Sand is exploitability. First of all, there are far more viable, threatening Pokemon with Swift Swim or Hydration than with Sand Throw or Sand Power. Sand Veil is very annoying, but not inherently broken (undesirable, yes). Secondly, aside from these abilities, Rain has three effects. It powers up Water moves, nerfs Fire moves, and makes Thunder and Hurricane 100% accurate. Even without the support of Swift Swim, this would still be an enormous offensive advantage for a dedicated Rain offense team. On the other hand, Sand has only two non-ability effects, causing 6.25% damage per turn and boosting the SpD of Rock-types. Considering that the only Rock-types of consequence are Tyranitar, Aerodactyl and Terakion, this is quite modest in its impact. In other words, Sand is only strongly abusable (i.e. abusable to the point of being potentially broken) by two Pokemon, Doryuuzu and Randorosu. If perma-Rain is banned (which in my opinion it certainly should be), could anyone seriously foresee problems with a Sand team without Doryuuzu or Randorosu? Even without Manaphy OR Swift Swim, I would still not want to face a dedicated Rain team.