It is hardly an advantage. You take an expensive UWA and convert it to something like 28mm with lower resolution everywhere but better borders. On FF, you can just use your garden variety zoom to do much better.
Is there not something to be said for a more even resolution across the frame and less vignetting? The Canon 10-22 also has much better barrel distortion than the 17-40 used on crop or full frame for that matter. You might be able to use your 'garden variety' zoom on FF to do better, but that FF body you are using will have cost you more in the first place. Cost is not irrelevent. I would be using FF for landscape if I could afford it, but even the 6D is out of my reach at the moment. Couple that with the cost of a decent wideangle lens to use with it. I own the 17-40 but at it's wide end it's not a fantastic performer on FF unless stopped down a fair bit. So just looking at Canon glass, because I PERSONALLY don't want to use third party lenses, what are my options?
Canon 14 and 24 L primes. 24-70 f2.8/4 L Zooms, all way in excess of £1000. Same for the 16-35 L which isn't massively better than the 17-40 at equivalent apertures. The only other option is the 24-105 which is cheaper I grant you, but not ultrawide.