I've played around with 3 under out of curiosity but never really given it a fair chance to affect my shooting because I didn't properly tune my setup for it.
Watching some of the top shooters (I am not in that group) at the recent Okla Longbow Championships I would say it's sure not required to shoot well but can be a benefit for some. Like a3dhunter said, especially at longer distances.
I shoot 3 under for accuracy but prefer split for arrow control. The arrow is closer to your eye and for close distances (40 yards and under) it is more accurate for gapping. With split I think distances past 40 yards might be easier gapping. It also depends on which finger you anchor with.