What is the down side to carrying an ID?

I always carry a little cash. I carry a credit card, in case something happens where my $20 is insufficient. I carry my ID both to be able to use the credit card *and* in the event that my unconscious self ends up in an ER. I carry all these and my cellphone in a plastic bag in my jersey pocket -- this way they are on me, don't get sweat upon, and I know they will be discovered if I'm an unfortunate crash victim. I also have my husband's number under the label "ICE" (In Case of Emergency) in my cell -- a more and more common convention.

Keep your ID on you, not your bike. In our ER, the only accessories that have ever come with an injured pedestrian, cyclist, motorcyclist, or etc. has been a helmet to illustrate damage. Never the saddlebag or seatpost pouch, never the computer, never the ID from the bike. The cops might have those, but getting the info to me so I can find a next of kin is near impossible.

Keep an ID on you -- any kind -- if not because of the law, because it will help you and those you love in case of badness.