Gmail has already distanced itself from web standards in order to differentiate and offer significant functionality improvements over its competitors, with labels being the obvious example. This API simply exposes these features to third-party clients, like Microsoft did with Office 365 earlier this year.
Although right now Google is adamant that ‘full-fledged’ clients should continue to use IMAP, the writing is on the wall. Google doesn’t need to cut off IMAP support in one swift action. At some point, Gmail will outgrow IMAP so much that integration through the API is the only viable option for alternative email clients to be competitive with web Gmail.
I don’t think people should be upset that Google is abandoning standards email. It’s just like how browser vendors move faster than the W3C. This is how the world gets better, pushing beyond the stagnant slow-moving standards bodies. Be thankful that Google is offering third-parties any access to the proprietary layers at all.