I'm also active in another forum. There, accounts are deleted if they exist for more than three months and still have 0 posts. However, I don't know if there is a technical solution that is doing this or if someone has to sit down once in a while and delete all these accounts manually.
I think it is sensible to delete accounts with 0 posts after some time. For reading only, you don't need to register. And if someone rediscovers the forum and finally does want to post, it is no problem to create an account again - when you didn't have any posts or PMs, you don't lose anything. Is there a way to check if a user with 0 posts has any PMs?