I'm sure this is an idea that BT would like to take on so they could screw much more money out of the customer for internet access.
We already are behind many other countries when it comes to broadband and this won't help. I doubt very much that a really light user who reads a few text emails and does a bit of browsing would need to have a broadband connection.
This is a giant step backwards IMO ... let's hope it doesn't catch on otherwise we could all be paying per Gb/ per minute!