Does it really matter?
* Takes more space to store the DB.
* Slower DB queries.
20,000 records is nothing from a database perspective. The time required to download any given webpage on here is probably 100x slower than the extra time those records put onto a query. The size those records take up is most likely tiny - 10KB sort of stuff.
* Harder to browse member names.
Why would anyone browse member names?
* Greater chance your nick is already taken.
Use a different name then? Sorry but nickname 'ownership' really isn't an issue IMO.
* Better shows how many actual member this forum have.
You seem to have found it out pretty easily. Does the number of users on the forum really matter?
As Raccoon pointed out, registering means users can keep settings etc.. Unless it's actually creating an issue, like there are only 25,000 possible usernames that can be registered (which I'm sure there isn't) or unless there actually is
a speed/size problem concerning the database (which I'm reasonably sure there isn't) then I just don't see what the big deal is.