There are still a few details, such as not being able to do https through your own domain (and apparently even for this new feature your app can only receive e-mail at your appspot domain) that may still make having a good appspot name important.

I guess if you had some clients to that app still out there (say, installed apps) somewhere, server gets deleted, but clients then try to connect to it - in the meantime someone has snapped up the name/URL and is collecting data from unsuspecting clients.

At least that is what popped into my mind.