Most of these icon sizes are provided by the developer only optionally. Any sizes not provided are generated on the fly by resizing with some kind of bilinear filter. If you're not doing any better than that bilinear filter, why provide the files?
Uhh, somewhat handy I guess, but it's really noticeable when a developer just resizes their icon to the iPad size, for example. When I see it, it makes me think of the app as lower quality.