Having tried this approach for close to a year, I can unequivocally say that it is *NOT* an acceptable solution. I don't know how those MTB kids do it -- maybe it has something to do with their being lower to the ground, or seated farther behind the bottom bracket? -- but on a road bike, most MTB shorts constantly flap against your top tube. Really freakin' annoying.

Plus they're about as aero as a drag chute.