I just put a mountain/'cross bike together for my 17y.o. daughter to ride in some of the Verge series (she had been riding 'cross on a Cannondale mtb). I found a not-too-old Scott rigid mountain bike, pulled it apart and rebuilt it with a mix of the components that came with the Scott and road bike parts. Added 1.5" tires, new chain and a tighter cassette, she tried it on one short test ride and we headed to the Crank Brother's U.S. Gran Prix last weekend. Guess the bike (oh yeah, let's not forget the rider) worked pretty well, since this was her 4th cross race and she placed 7th on Saturday and 5th on Sunday in women's B.
So, the mtb to cross bike can work and pretty well, although if she keeps this up I can see a true cyclocross bike under her next year.