I tried commuting on my mtb and hated it. Couldn't stand the knobby tires, the twist shifters, the straight bars. I also didn't like having to put the rack on it to carry panniers, since I didn't want to ride with the rack while actually mountain biking.
So I got a hybrid-like bike (rivendell glorius).
I'd ride the cyclocross bike if that was my other choice.
2007 Rivendell Glorius/Trico gel with cutout (not made any more apparently)
2005 Specialized Sequoia Comp/Specialized Dolce
2006 Kona Cinder Cone/another Trico gel
1986? Bridgestone mixte/Brooks B72
1991 Bridgestone 300 Xtracycle/Terry Gelissimo