Granting that level of access with no fine-grained control to 3rd party apps seems insane to me. I predict at least a couple major security incidents in the future.
Not sure why they really need an API though. Seems to me like it would have been a better solution to stick to the IMAP protocol and allow an alternative method of authorization. For example an application would request access to your email, then they'd get an access token and use that to authenticate with IMAP. They could then try to delete an email with the IMAP protocol and if they hadn't requested that scope they're receive an error.
HTTPifying everything seems to be a trend, not sure what the real purpose is - if anything it's just making things less open.
This was the reason Google provided for not supporting Git in Google Code initially. They do support git now though.
In fact, at least the connotations I have with IMAP/SMTP: "shit still gotta learn that before I can start". Even though it would probably not even take a couple hours to get started.