Putting an optical drive in a system costs money and also makes the device larger/bulkier. Just look at netbooks. No optical drive, and they can be incredibly small. If the 360 had no need for an optical drive, it would be quite a bit smaller.
I just think that if Sony and Microsoft can stretch this cycle out all the way till about 2014 (which sounds crazy I know...), then they will say F U to the idea of having an optical drive. Especially Microsoft. If they had an optical drive in their next system, it would have to be Blu Ray. They would like to avoid that if at all possible, and I think digital download is the way to do that. Obviously, because some people aren't going to be able to download these gigantic files, they are going to have to find a way to service those customers as well, and downloading kiosks or something will be the way they do it.
If MS releases a console prior to November 2014, then maybe they will still have to have some type of drive, but if they can hold out until late 2014, I think they can actually get away with it. It would be a bold move for sure, but I think they just might take the chance.