Forgive my ignorance: I get that it uses the extensors instead of the flexors, and it doesn't go through the carpal tunnel, but wouldn't you just eventually get RSI in another part of your wrist then?
Lots of stuff goes through a small area in the carpal tunnel so it's easy for it to all rub too much and get irritated. My understanding is that the extensors don't all go through a similar narrow area so are less likely to have this problem.
The flexors are used a lot for typing. By moving some of the wear from mousing to the extensors, the idea is to better spread out the wear and tear.