I had a similar thought about this the other week.
One of my mates at uni has a standard off the shelf MTB (will have cost him about £250 at a guess) but he won't take it anywhere it might get dirty. I asked him why he didn't buy a road bike, seeing as he only rides on roads it'd be the much more sensible option. His response was that 'he didn't buy a mountain bike, he just bought a bike'.
Now I don't know how much cheap road bikes are but if somebody is buying a bike why has it now become standard to buy a mountain bike (even for road use)?
I got my first road bike last year when I was 20 but when I was younger I had bikes for jumping and the likes so never saw much use for having a road bike.