Depends where you're riding I think.
My part of the world has fairly flat, gravelled paths pretty much everywhere, and any long off road ride is bound to take in some tarmac too, so it's great with a few close ratios at the top end, like on my cross bike.
When it does get gnarly, steep and technical though, it's good to have smaller steps between the ratios at the bottom end too.
I seriously considered running a 12-27 10 speed cassette with thumbies and a DA long-cage mech on my Pace, but liked the new XT rapidfires and Shadow rear mech so much I decided against.
10 speed chains seem to run fine on cyclocross bikes so can't see them being problematic on XC MTBs.