Defeats the point of SatNav?
Sounds like a rubbish idea. If I'm coming up to the crest of a hill and can't see what's coming up, I can look at my SatNav to find out that there's a killer bend just over the hill. With a 'true 3d' SatNav I won't be able to see over the crest of the computer-generated hill either.
Also, the landmarks stop you from seeing the details of roads off to the left and the right, and surely if you can see the landmarks on the SatNav you'll be able to see them for real anyway? Maybe it would help in the route planning stage but it seems pointless to show this stuff while you're driving.