The thing that is the most annoying about the same username on different servers is that most of the time only the username part is shown (for example on feeds and comments). So a troll could make accounts with the same username on a different instance and discredit the original comment. For example, you are kadu@lemmy.world, I could create kadu@lemmy.ml and say stuff here, and both of us would just show up as "kadu" and to others it might seem that we are the same account without going into our profile to see our full "address".