Mostly an echo of what has been said, but put slightly differently:
Lens sharpness is better stopped down, but if your shutter speed is too
slow, your picture will be so blurry that it won't matter.
I can't speak for others, but I like fast lenses because
1) they allow fast shutter speeds or adequate shutter speeds in low light, and
2) they give a narrow DOF to isolate the subject and blur the background.
Originally Posted by Bombsight
We don't always get a better picture. In ideal circumstances, we get a sharper picture. But circumstances are not always ideal, and sharpness is not always the only thing one cares about. With a high quality fast lens, you may need to be a pixel-peeper to see
the sharpness difference between f/2.8 and f/8, but you can see the
difference in background blur in a 4x6 print.
If you shoot only still subjects with a tripod or in good light and you don't need to blur the background, then by all means buy only slow lenses and stop them down to the diffraction limit or beyond.
Faster lenses (at least f/2.8) also allow the af to work better, but I doubt there are many shelling out big bucks for that reason alone.
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