I semi-disagree. As long as you don't seriously cross-chain (and really, how many people use the big ring on an MTB unless they're in an almost-spun-out situation anyway?), you should do fine with a medium cage.

Example: a medium cage derailleur with a capacity of 38. Front difference of a 44-32-22 is 22; rear difference of a 12-32 (typical 8-speed) is 20. Sum is 42 - - not far enough off 38 (IMO) to where you can't easily fudge it if you don't do silly things.