exactly. the lines really get blurred when you start putting all the parts that make a road bike a road bike (high pressure slicks, rigid forks, up-sized gearing, drop bars, etc.) on to your old hardtail MTB frame. that's exactly what my back-up MTB commuter is. it's got slicks (for 3 seasons anyway, i still roll fat knobby studs for winter), a rigid fork, bullhorns, and the largest crankset i could realistically fit on the frame (a 48-38-28 triple). it runs great and performs marvelously well as my winter/foul weather/back-up commuter, but it's still 11 pounds heavier and a couple mph slower than my primary fair weather commuter road bike.

what is this bike? (pictured below with winter studs, not summertime slicks)



it's not a road bike - too heavy (28 lbs.), longish wheelbase, 26" wheels
it's not really a mountain bike anymore - bullhorns and rigid fork
it best fits as some kind of hybrid, but even then it's still an odd duck.