If you actually run out of 223 yet have plenty of crossbow bolts available, I think your priorities need to be evaluated. :D
Well, theoretically you can use the crossbow against the occasional sole or small group of zombies, and reuse the bolts. It's more difficult to reuse a .223 round.
Then there's the whole 'no noise to attract MORE zombies' benefit.
I don't know if one needs the wear cycles on your high end AR trigger,
when a nice purpose built crossbow complete with trigger and butt stock is about 6-800 bucks cheaper
I initially thought the thing was a joke. Especially when you get to the attachments part.
Then again, the attachments part did it's job - it got everybody posting it everywhere, thus should generate SOME sales. If not of THAT crossbow, but of other things they also sell.