[enomar]

It's unfortunate that once deleted, names can't be reused by someone else. I imagine this has something to do with their security model.

[timdorr]

Simple: You don't want a malicious developer snapping up old app names and replacing them with phishing sites.

[amichail]

Why is the name important? Won't you be connecting it to your own domain name anyway?

[gcheong]

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.

[tomjen2]

It shouldn't be too difficult to simply set up your own email and then forward it to the app email.

[michaelneale]

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.