I'm certain Nintendo never gave a second thought to what Drizzle Politoed or Drought Ninetales would mean to the 6v6 singles metagame, simply because Nintendo has never, ever cared about competitive 6v6 singles.
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.
Finally, Drizzle teams are inherently more balanced and less vulnerable to type disadvantages than are Sand Stream teams. The Swift Swimmers lack common weaknesses (for instance, Kingdra and Ludicolo are not weak to Electric or Grass), unlike Rain teams with tend to be very weak to Water, Ice and generally any very threatening Pokemon holding a Balloon. Moreover, there is a far greater type range that can exploit Rain: Electric Pokemon benefit heavily from the use of Thunder, and make excellent complements to the Water types, Fire-weak Pokemon like Scizor and Nattorei appreciate the weakening of Fire moves. Sand Stream teams are very powerful, but they are so because of exactly two Pokemon, and even so they have serious liabilities. Drizzle teams are much more powerful because there is such a wide range of Pokemon that can exploit Rain, going well beyond just the Swift Swimmers. Unless the team you are facing is a dedicated anti-Rain team (by which I mean something like either Hail stall, or offense with Nattorei, Porygon2 and Tyranitar all on the same team), you will always have a significant advantage while using a Drizzle team.
Since we're on the topic of Weather, I think the jury is still out regarding Drought teams. Those tend to have many more liabilities than the other two Weathers, such as difficulty facing Dragon-types, being much more vulnerable to the type of attacks Sun promotes (Fire), and generally the poor coverage options of Grass types. I would rather wait and see what happens with Drizzle and with Doryuuzu and Randorosu before I worry much about Drought at all.
Edit: Yes, I realize that both Rain and Sand nerf Morning Sun and Moonlight as well, but I don't think many Pokemon are using those anyway.