I'm the kind of person who keeps stuff too long. Stuff I shouldn't have bought, stuff I should have sold long ago while it was still worth something (I'm talking mostly music/recording gear). I've heard this thought about cloths: if a year has gone by without it being worn, then get rid of it...that could probably apply to a lot of things.
You could force yourself to ride the mtb a few times and see if you find anything appealing about it that you might have forgotten. Does it fit you well? Can it be adapted to be useful to you, have you tried slick tires on it (which would make it much more faster (and roadbike-like) than knobby tires). Would a cyclocross bike be that much of a better ride than the mtb or do you just want an excuse to get another bike? If you honestly dont want to ride it anymore (and don't even want to keep it as a back-up bike) then let it go.