I want my email to be a consistent experience - not to get some different behavior if I access through a certain device. Email and its simplicity was universal - I saw the same text on my phone, tablet, laptop and desktop. Will every mail sender who makes use of this, include the JSON and an alternative link to achieve the same result. I think this could get very inconsistent very quickly. Fragmenting email is not something I like the idea of.
For Google Apps customers' internal mails I think this is useful. For the wider Internet I don't like it.