It's just a matter of time for the first 36" MTB's to surface. Right now rims and tires available are pretty low-end, but once that's taken care of, weight penalty would be such that the BIG experience will not be spoiled by weight too much.
And you thought a 29"er rolled smoothly over obstacles...
36" is practical only for riders 6'1"+, or really extreme geometries need to be tried.
For my 6'4" I am convinced a fitting 36"er with 195mm or so cranks can be made that fits me well, has not toe overlap and handles as well as the average AM 26"er. Doable on tight trails, a blast when given a bit of breathing room, and bumpy terrain. But also over soft soil it should be killer.