said by tshirt:A good question, Maybe Jason can answer for certain.
I'd guess that because D2 is closer to D3 than D1.1, but was never widely adopted, it can co-exist for the meantime, which may not be more then a year or two which is why he suggested a D3 modem, and probably should have hinted at IPv6 CERTIFIED (per comcast's list) for best future longevitity (only the future is future-proof)
Unless someone is happy to be on the Economy tier for a long time you are going to need to do channel bonding to take full advantage of your speeds in the future, and that means you want a D3 device. You also want that investment to last a long time and not learn that your D2 device is in End of Life or End Of Service status in a year or two. You also want modern features like IPv6, and to know that the vendor is still releasing firmware updates periodically.