I have actually seen myself scanning my sketch and just cleaning that up, getting rid of all unwanted marks and just using that sketch instead of lining over the top. It depends, but yes, the energy is important, esp for comic work where you want the characters to seem alive like they're moving or talking etc.
For one off illustration I've been stuck sometimes with what to draw, but with a comic because comics are literal drawings of what's going on so you don't have to think too hard just draw what the script says. It's a lot of fun
I enjoy sketching, but for practicality and being able to convert to digital, the sketching is just too much of a clusterf*** of smudges and hashes and what not it's usually too much of a mess for me to do anything unless I take it to the lightbox and trace it onto a new sheet and make a clean version. So to sum up, I like sketching, but the sketches themselves aren't too useful so I just stick to lineart and flat colors for what I'm working on for the most part.
Like you said, it depends on the piece itself. There are some sketch artists who are fantastic without color. But I've found that for the most part, people tend to like looking at colorful images. I know I do xD
Line art I don't find particularly attractive (which as someone before me just said, is not the same as a sketch). Line art is just a simple black and white outline of a character, and unless it is fantastically done, it's not as fun to look at. At least with a sketch you can get some shading in or something. That's just me though.