While the extension does offer some added flexibility, there's likely a very pragmatic reason behind it: the longer you have to restore your account, the more likely you are to come back. People who deleted their accounts in a moment of outrage (say, after a privacy breach) might not feel like they have to commit to that decision knowing their data will still be waiting for them weeks later.

There is a downside. If you are bent on deleting your account, Facebook won't truly scrub your data for another two weeks beyond its previous grace period. You're not very likely to see a breach during that interval, but it could be frustrating if you wanted to wash your hands of the service.