Another reason why not to sharpen at the very beginning already is: The impression of sharpness also depends on the viewing distance. If you sharpen an image at full resolution and then scale it down to 1/4 of the resolution, e.g. for the web, then the effect of sharpening will hardly be visible anymore. For example, you have pixels with the following values:
which might look like this after sharpening (the local contrast increases):
Then you scale it down to half the size, which would result in a single pixel with the value
(mean of the above). This is the same result you get with the unsharpened image. The point is, you also get a «downscaled» version of your image when watching it from greater distance (here's the link now :)). If the image is printed with 600 dpi, but my eye can only resolve 150 dpi from the position I watch it, then it can be sharpened stronger.
Also, sharpening is always a «lossy» operation, i.e. you try to guess what the original image looked like without really knowing it. And finally, sharpening can increase noise and compression artifacts, and it's usually a good idea to postpone all kind of quality loss as far as possible :)