I think that both abilities should be allowed, for these reasons:
First, the abilities only increase evasion by 20% in sand or hail, meaning that roughly 80% of the time they won't help that Pokemon. Also, the abilities ONLY work in their respective weather. Since all four weather conditions are viable, and all four now have Pokemon that can set up their perma-weather (for example, Ninetales has Drought, which sets up perma-sun), in reality its more than an 80% chance that these abilities will not affect the game. So, I don't see why these abilities should be banned, since they won't affect games too much. Sure, one miss can greatly change the game at times, but I don't think that these abilities are game breaking.
Also, without Snow Cloak or Sand Veil, some Pokemon get unfair move restrictions. the main Pokemon that I'm thinking about are Cacturne and Froslass. Both of them get Sand Veil / Snow Cloak as their 'normal' ability, and a Dream World ability. If Cacturne and Froslass can't use Sand Veil and Snow Cloak, then they would lose out on some good moves (or their egg moves and 4th generation moves). Cacturne wouldn't be able to use moves like Seed Bomb, Nasty Plot, Disable, Focus Punch, Drain Punch, and Dark Pulse, while Froslass wouldn't be able to use moves like Spikes, Disable, Ice Punch, Trick, and Pain Split. I don't see why we should 'punish' these Pokemon, because they have abilities that usually won't affect battles.
Finally, I'm all for banning some things though. I would have the ban Brightpowder + Snow Cloak (and other similar combinations) if it isn't there already, and I could understand banning Sand Veil Garchomp, since its that good.