After all, purged should mean that it no longer has any association with its previous owner, and in this way it's like any regular unclaimed username.

But it is this the case? AFAIK comments, community posts, poll responses, presence on other people's profiles and such from purged accounts don't vanish (at least they don't on LJ), that is only broken if the name is reclaimed, and that causes the extra effort that you have to pay for. And that would still be true if this was to become a community.