This question sounds a little too close to "nothing to hide, nothing to fear" to me, but in any case I think it's Facebook, attempting to build dossiers on billions of people for profit, who need to justify themselves; not me for wanting to remain undocumented.

As far as concrete reasons go, I've had to deal with far too much fallout from being incorrectly flagged by braindead processes trawling private databases which I didn't even know I was in. Since lots of these databases share information, but not necessarily updated corrections, I still run into the same mis-flagging every few years, across utilities, courts, credit agencies, banks, letting agents, etc.

As far as Facebook goes, being a citizen of the CCTV-riddled UK makes me acutely aware of the power, and potential abuse, that facial recognition technology can bring; having images of my face tagged and fed into a database does not sit well with me.

Since I don't use Facebook, I don't even get the meagre upside of whatever services they build on top of this database (some kind of gallery, I presume).