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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.
At f/4 it's the worst point of the lens, but it doesn't really become useable until f/11 which is quite a small aperture.
The newer high pixel count FF cameras really show this effect, and if Canon release the 1Ds MkIV with a 39MP sensor it'll be even worse as the centre of the image will be high res and the borders won't! If you imagine a camera like the 5D with 12.8 MP the resolviing power across the image is more or less the same so it's not as noticeable, however the MkII with almost double the pixel count is out resolving the lens at the borders noticeable in the image.
Wide angle lenses probably show this problem more than any other, the 70 - 200mm f/4 is apparantly much better, but as Photozone say it's much less important in a telephoto lens than in a wide angle.
Canon appear to have found it difficult to design a lens to compete with Nikons 14 - 24mm however they have recently applied for a patent on a new wide angle design which appears from the drawings to resemble the Nikon in having a bulbous front element, I'm sure that this will perform better than the current model.
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