f/4 isn't particularly large as apertures go but imagine a table setting where you want some in focus and the background blurred as an example, How about a stack of shopping trolleys where the logo of the store is in sharp focus and the rest blur nicely away? So long as the scene is relatively distant the depth of field at 17mm f/4 should be sufficient, so you might want to use a wide aperture in low light hand held. I did some architechture recently (with a different lens) that could easily have been done at f/4, I actually did it at about 28mm f/8 and the fall off in resolution towards the borders is still noticeable, though not anythig like as much.
Is a photograph of a flat subject shot parallel to the plane of focus representative of the results you'd want from these scenarios? It doesn't seem that way to me, one has no subject matter beyond the focal plane and the others rely on it. As mentioned, if you want closer to perfect results then a prime is the way to go as the compromises made to allow the zoom aren't there. However, even with a prime they're going to be more uniformly sharp with apertures approaching f/8 than they are closer to wide open. Also as mentioned, the 17-40 is pretty old and wasn't designed with the current ultra-high res full frame bodies in mind, and it's not really surprising that the new FF sensors are out-resolving it (along with a few other older lenses). If you don't want to use a prime then you might want to consider a 16-35 f/2L mark 2 which was built with edge sharpness and light falloff in mind in order to overcome the deficiencies found in the mark 1, and does a lot better than the 17-40 around the edges.
The flat subject parallel to the focus plane is perhaps the best way to illustrate the resolution fall off, if there is detail in a third plane then the possibility of field curvature begins to intrude.
Unfortunately Prime lenses are not immune to this either Canons 20mm f/2.8 is a particularly poor performer, but all lenses show it to a greater or lesser extent, and high resolution FF sensors make the problem worse.
Funny but when I posted this on one of the pro forums they had seen it and understood that with higher MP sensors comming the problem will be more noticeable as centre resolution will continue to rise while borders will stay broadly similar. Smaller resolution cameras such as the 5D MkI or the Nikon D3 don't show this effect so much as the resolution is maintained across the frame more evenly.
The real issue is that sensors have evolved much more quickly than lenses have, even recently designed lenses are showing this problem, maybe the whole DSLR system needs a rethink before it moves forward again