This seems to clutter the system, and i didn't see a given rationale in the FAQ.
The rationale for the patent numbers is in fact in a Stack Exchange blog post: A Stack Exchange To Prevent Bad Patents
Ben Brocka is pretty much right that searchability is the big thing.
In particular, this allows for interested parties, like say... patent examiners, to instantly check to see if a patent they're reviewing has any prior art or other information here without having to sort through other things.
Also, giving the patent number allows review to see if there is art here listed that is earlier than the patent. Say an examiner or defense attorney has a nice reference patent to cite as prior art, except that it's NOT PRIOR. Searching here on that patent number might surface some prior art uncited in the patent but listed here. Could be key in the right case and might be missed without that number.