The only reason people tend to emphasize the importance of doing sketches is that some people seem to think that good pixel art is made by people studying lots of pixel art, which is simply not true for the vast majority of pixel artists.

I think a lot of people actually do that, mainly looking at other pixelart to get inspiration. Which is why there is so much style inbreeding going on. Examples being the sword and sworcery style being copied all over the place, and an older example are Seiken Densetsu like trees. I personally find that it is always better to look at real life for reference and for inspiration look outside of your own medium. As far as pixeltech and things like palette handling goes, that is where you look at game art. Just my 2 cents on this small part of this topic.