When I switched to full frame I ran the calculations and found that for EVERY SINGLE EF-S lens from focal lengths 10mm to 250mm (16mm-400mm equivalent) it's full frame equivalent lens would be always substantially much better, almost always cheaper, could acheive better subject isolation with shallower DOF and be much much sharper. In fact full frame lenses typically have twice the resolution of their crop counterparts.
Example:
10-22mm vs 17-40mm = lens is cheaper, has 202% the resolution, has 1-2 stops shallower DOF and has a longer reach on the long end, and is weather sealed.
17-55mm IS vs 24-105mm IS = lens is cheaper, has 205% the resolution, has 0.5 stops shallower DOF, and is both longer and wider, and is weather sealed.
55-250mm IS vs 70-300mm IS = lens is only $150 more, has 186% the resolution, has 1-1.5 stops shallower DOF, is wider, but also only has 300mm vs 400mm of reach, HOWEVER it is so much sharper that cropping it actually results in a 6% sharper image which has shallower DOF.
18-200mm IS vs Tamron 28-300mm XR (full frame superzoom) = lens is $100 less, has virtually identical resolution, has a 1 stops shallower DOF, is slightly wider but is slightly shorter (29 vs 28mm 320 vs 300mm).
Remember crop lenses need to have their focal length multiplied by 1.6 and 1.5 stops subtracted from their aperture to compare them to full frame lenses. So a 17-55mm f/2.8 lens is the equivalent of a full frame 28-90mm f/4.5.
The point is that there is zero image quality benefit and it would cost more to use EF-S lenses on full frame than their EF counterparts. It's pointless and engineering a body with this function would also be pointless.
Nikon's dogma as a company is compatabilty that's why you can use 50 year old lenses on their bodies which still work. Even if that compatability is pointless.
Keep in mind my argument is before you consider that you'd be working with very few megapixels if you were to make EF-S lenses compatible with full frame bodies. That really sinks the whole idea.
The only major argument for being able to use EF-S lenses on full frame is lens sharing. However if you actually study the possibility of lens sharing it's much better to share EF lenses between a crop and full frame than EF-S lenses between a crop and full frame.
EF lenses on crop tend to be about 10% worse in resolution than their EF-S counterparts.
So if you were to share EF-S lenses you'd lose 50% of your resolution just at the lens nevermind the megapixels on one body. If you share EF lenses you only lose 10%. I'm pretty sure most people would much rather have 10% less resolution than 50% so it's much much much better to only have EF lenses and share those with your crop than to have EF-S lenses and share those with your full frame instead of getting the EF counterparts. Enabling people to share EF-S lenses between their full frame and crop cameras would be pointless, as it's just giving them the option to have much worse and more expensive gear.
Hope that clears things up. If anything making EF-S lenses incompatible with full frame motivates people to not use EF-S lenses on full frame which is better and cheaper for them in every way.
Hope that helps.
