I actually saw some pics from a DX glass on a FX camera, and honestly it came out much better than expected. You could get away w/ a 5:4 crop and probably have very minimal vignetting.
I put on my Sigma 30mm f/1.4 (crop lens, but works w/ FF). The vignetting.. :biggrin:
When you crop down this photo so that there is no more vignetting, you lose 40% of the original resolution. It's easy to crop all photos the same way using Lightroom, which every photographer should be using anyway.
On a 20MP 6D, you would end up with 12MP, which can get you a good 11"x14" print. For me 12MP is certainly enough.
How many DSLR hobbyists actually print? I would guess that most people just take photos and post them online. For professionals this is perhaps an issue, but then again most established professionals, especially ones actually needing to shoot for print, should have the budget to go FF easily.
The fact that you can't use crop lenses on a 6D is a big "f-you" from Canon. If you're using a crop and want to move to a FF for cheap, the 6D is NOT for you because then you have to spend an extra $2000+ immediately for all FF lenses.
The Nikon D600 is the same price as the 6D, has more megapixels, a crop mode that offers more resolution as a result, resolves more details in video than a 5D MK2, more AF points, faster shooting, better ergonomics, compact, has a built-in flash, a scratch resistant glass LCD screen, and ability to use excellent crop lenses to soften the blow during conversion.
Not to mention using crop lenses for shooting video would not have any issues. Video's only 2MP anyway. On the 6D you can't even use your crop lenses to shoot 2MP videos.
I could start using my D600 immediately for work and play with all my current crop factor lenses. Keep on generating income with the new body. If I was going to FF on Canon, the 6D would be a $2000 paperweight until I recovered from the initial $2000 purchase and scrambled together enough money to convert my entire lens lineup to FF.