The email standard was built so that you can check your email with a client, and view the contents offline at your convenience, archive, back up, etc.

Google's email proposal kills this functionality.

When was the last time I used this, you may ask? On Saturday. I was in one of the slot canyons in Utah with zero signal, and, while taking a break, I wrote several emails in my Opera email client, in response to some messages I've procrastinated writing a response to.

The trickery is that the sender might think they're sending a message over email to the recipient, whereas they are sending the message to Google's servers over an internal protocol, and are sending a link to the message to the recipient over email.