http://img96.imageshack.us/img96/8020/iosicons.png
http://img404.imageshack.us/img404/6851/osxicons.png
The iOS ones aren't the best example - it's not a complex icon - but you can still see the hand crafted ones are better, e.g. some of the lines on the rescaled items are not the desired 1 or 2 px widths and so look blurry (e.g. the ipad icon); you don't have the texture; and you lose the corner line definition (e.g. see ipad settings).
The OSx ones are better examples as it's more complex. The scaled icons look rubbish.
We're not there yet. You still need to tweak icons so they look good at various sizes - subtle things like 1px lines or highlights still need to manual tweaking. Pixels too small to see doesn't make the problem go away, you still want to optimise for resolution so you match the pixel grid for sharp definition. Some people don't have that attention to detail (and those devs can just rescale them) but many do - and personally I think it's worth it. And I appreciate having the opportunity to hand craft rather than have the OS just scale it for me.