Look at just about every other forum site in the internet; the exact same thing happens to them. It's their choice not to use their accounts; some people make an account in case they want to post later, or because they just want to lurk (which means just look through the site without posting anything). There is a good chance they simply forget about the site after making an account!
Also, deleting those accounts is pointless for these reasons:
- Those accounts are not harming us in any way. Why make PM do the work to delete them? There is no point in it at all. He could spend that time on something worthwhile.
- What if they come back later? I know plenty of people who have come back after several months of inactivity.
Also, some of them actually have made posts, they were just rejected/hidden for breaking a rule. PM has also stated he won't delete accounts, so basically the answer is no.