It just seems like a way to add a point of failure where there doesn't necessarily need to be one. Vendors need to make a recognizable brand name for themselves, of course, but buyers don't. I know several people in real life who use the darknet markets, and I know for a fact that several of them reuse usernames for the darknet markets that they use on the clearnet as well.
Why not just have a numerical user ID instead of a username? Like how ICQ works. Do usernames actually offer any advantage in this scenario? Market admins already force people to use PGP for their own safety, why not this too?
Usernames are there so you can easily remember them, I see your point here but lots of people wouldn't note them down and forget them which means a lot of support messages because of people being locked out of their BTC filled accounts.
Buyers sometimes also need to be recognizable by vendors so that they know who are returning customers, to do this with numerical ID's would mean storing every single customer's unique ID who have bought from them, which is a terrible idea, since even after order history is purged, they would essentially be keeping a list still.
You don't necessarily need to use a different username on each market too but it essentially removes the link between purchases on multiple markets and if someone phishes your password they can't go through each market and steal every account.