if there were any sites printing fake BP's though, TSA would shut them down in a heartbeat!

also, we could just be talking a situation where a woman (or man) books under her married name even though her ID still shows her maiden name.

or when someone has been using an alias for a long time but has never changed their ID and they don't want their associates to know about this.

or when a transgender person's ID still shows "M" for their boarding pass says "F".

there are lots of other cases such as these.

In these cases it might be more convenient to "forget" your ID or hide it in your checked baggage. (Do the secondary screeners do anything if your "forgotten" ID is found in your carry-on baggage??? would they even know you had "forogtten" your ID?)