It's not the resolving power of lenses which is the problem, it's the border & corner performance. On a 12MP camera it's much less noticeable as the resolution across the frame is more or less uniform, it becomes an issue though as the MP count increases. The 5D MkII using a 17 - 40mm F/4 at the wide end wide open really shows this problem - plenty of resolution in the centre, no measureable resolution at all in the corners, it's a dramatic fall off which is really noticable. Up the MP count higher and it'll become even more noticeable as centre performance increases, while border & edge stays the same.
Not trying to rock the boat or start a debate, but I just wanted to get this off my chest... People like to pick apart Canon's ultrawides (i.e. 17-40, 16-35) for example that the corner sharpness is bad, especially at the extremes... Mkay... At the end of the day, what are you shooting in which corner sharpness, especially if used on a Full Frame camera, do you need that corner sharpness? architecture? Landscape? If you are, then why aren't you saving up to use a T/S lens which you would get that corner sharpness such as the new 17mm TS? That lens is geared for pro architecture/landscape photographers. 17-40, on a crop body, is kinda like a short stubby all around lens however on a full frame, it's as close to a fish eye and you get without the fisheye effect. That lens is a good lens but has it's place within the professional photographers bag. I know i'm one to talk and battle this battle on a daily occurance with the costs and everything, but in the end, using a 17-40 or even the 16-35 on a detail critical architecture shoot and then griping about corner sharpness, to me, is like someone using a hack saw to cut off branches off a tree and complaining it's taking to long... It can get the same job done but isn't the correct tool to use.