Not only does it force good nameing and scope practice apon you, but it also makes the compiled code better, (apparantly).
Yes, Mark mentioned that there are optimizations that could be used because BM no longer has to do as much checking/conversion (or something like that).
Personally, I think Mark should change the Superstrict keyword to strict, Alias Superstrict to Strict (for backwards compatibility) and do away with strict mode altogether.