I don't know what you mean. People are saying that Facebook wouldn't allow them to reuse their own old username. I think it's to prevent identity theft as someone could pick up any deleted username and pretend to be the old owner. They could even re-add the old owner's old connections to establish the scam. A few people might think "I thought I was already connected to this guy!" but most wouldn't think twice if they saw that the username and photo were the same.

I think it makes sense to allow reuse of usernames, but only after a sufficiently long period of time. I don't know if 20 years is enough, but something in that ballpark would be needed to reduce the risk of identity theft.