XO1 is stock Avid Shorty 4 cantis. There is a mounting hole on the front fork, so you could probably put the 105s on there. But there's no such hole on the rear, I think you're out of luck on that.
As for why: I could see wanting to replace the extremely noisy cantis with calipers if you had it set up for more-or-less road use with skinny tires (that's what I do, for better or worse). In fact, I've considered calipers several times, but the combination of those bare canti bosses looking really dorky, plus my inner frugality sends me back for one more tweak of the toe-in to try to kill the noise. Something about the XO1 fork + those Avids though . . . man those suckers can squeal like little piggies.