I know that many other forums disallow having several accounts. PC has, to my knowledge, never been strict on that, and a lot of people have alts. Some have them for fun, some have them for reserving names (every time a new gen of pokemon is revealed, the new pokémon names get snatched up within hours xD) and others maybe like to use one account for one part of the forum and one for another. Mods might like to have a non-staff account to log onto now and then to see the forums from a more relaxed point of view.
We might also be the only forum to never actively prune old accounts. There are an immense amount of accounts with join dates from early the last decade and 0 posts. But unless someone really craves their username, they will continue to exist.
Alternate accounts can be fun. It's a positive thing if PC is fun. To counter your proposal, what
harm is there to allowing multiple accounts?
One obvious answer is that people can snatch up nicknames and hold onto them but not really use them, but it's generally frowned upon to do so (or it should be) since we kind of want to be nice to each other here ;) But for those who really do want to hold onto nicknames, it's not really a negative thing.
EDIT: ninja'd
Let's not forget about spammers, trolls and idiots who get banned and have a dynamic IP address, because they can make an alt and evade the ban easily.
That's not too many people, to be honest, and if they're stirring up trouble anew we catch them again.