I have a couple Altus RD's that use the large pulleys. I suspect that the design allows the quick snappy shifts you get with a short cage (vs long cage) RD, but you get the extra chain wrap of a long cage RD. Brilliant idea, and I think it will catch on gradually, as more people run 32 and 34 tooth cassettes on their road bikes.

The Altus RD's are pretty heavy though, so I hope we start seeing the large pulleys on some higher end, lighter RD's.