Sandstorm, though it is no longer permanent, can be pretty safely assumed to be up the majority of the match on a team with a Sand Streamer. Both Sand Streamers are extremely durable and can find a ton of opportunities to switch in, and both are also extremely decent Pokes on their own (unlike Ninetales and Politoed). So, for Sand Rush Excadrill, finding sandy times to switch in shouldn't be much of a problem. Yes, it's hard to set up a Swords Dance, and no, it probably won't be able to sweep a whole team in one go, but I've had massive success just using Excadrill in "spurts" and usually killing at least one Pokemon before having to switch out.
With that said, Mold Breaker is one of the best abilities Excadrill could possibly receive. It definitely shouldn't be discounted, and I think it has a lot of potential regardless of whether it's scarfed or banded or given lefties or anything.
EDIT: Now that I've tried it, Scarfed Mold Breaker Excadrill is amazing. It smashes up so many more pokes than you expect it to. I actually like it more than sandstorm-reliant variants now just because Mold Breaker makes it so flexible. As of right now, it's my favorite scarf user in the OU metagame.