I've made it to the 20s with a weatherless team, which is the highest I've managed to reach this generation. Like people have mentioned earlier, sand hardly registers in my mind as weather because, with a few exceptions, it really doesn't give pokemon overpowering attacks. With Rain and Sun, the weatherless team has another advantage. No matter how hard people try to avoid it, Politoed and Ninetales are dead weight. They change the weather, try and hit hard or cripple with status, or help set-up but they really can't do much. So the weatherless team starts out effectively 6-5 against Sun and Rain. Then, its easy to plan ahead for these teams. When we start team-building, we think "I need to make sure I can take a boosted Hydro pump or flamethrower/flare blitz". And often these attacks are the most dangerous ones on these teams. This means, most teams already have these well covered. For weatherless teams, we can choose from a far greater range of moves and strategies to base our team around, and in a metagame where more pokemon are usable then any other, it's really difficult to counter every threat.