I think it's only worth doing if you're trying to get your handling "perfect". I went from a super long 100mm and 685mm bars to a 60mm and 785mm bars and I was wishing I'd done it sooner. This was about 6-12 months ago. It improved my descending greatly, but I find myself wishing the stem was a tiny bit longer for the steeper climbs and the bars a little bit shorter for more general riding. Consequently, I'm thinking about going to 70mm and 750mm bars for general use and using the 785/60mm combo for DH only.
If a 70mm stem shows up at the right price then I'd give it a shot. Your circumstances may vary.
EDIT: Actually, I'd probably go shorter stem for DH. Maybe a 50/55mm even.