Somebody could maliciously take several login ids (all you need are several e-mail addresses!!) and delete all of them thereby preventing them to be used ever by anybody. That would not help wordpress.com.
Wrong. It would help wordpress.com, because they could claim x number of registrations without having to suffer the extra server traffic of actual blogs. It would not help potential wordpress.com users, but many names will already have been snaffled by people who just wanted an API key and have no intention of blogging here, and most people are used to not getting their desired usernames anyway.
Having said that, I do agree that the 'never re-use a deleted username' policy is going to cause some frustration among users. Livejournal's practice of final deletion a month after the initial request, after which the name is released for re-registration, seems to me a sensible one; though of course I don't know whether it would be possible or desirable to replicate that here.