Nice, but I'm very reluctant to give any third-party access to my mailbox. Most sites I use let me reset my password by email, so this is like handing over the keys to the castle.
However - how many people trust our email to small web hosting companies? How much better is that?
For example - many of my clients use the mailbox provided by Webfaction who I'm sure are beyond reproach but do I trust them any more than I trust a well known SASS company that integrates using the GMail API?
I run a small self-tracking service (zenobase.com), and would love to let my users pull in some "metadata" from their email (e.g. the number of messages in the inbox, or the number of messages sent by hour). But I don't want my users to trust me with unrestricted access to their email.
Being able to replace my current IMAP client code - full of hacks to convert internal ids to Gmail ids and format conversion issues - into a couple of HTTP calls sounds great to me.