Ollie Johnston said "Draw clear, not clean". When a drawing isn't working it's always tempting to clean it up in an attempt to "fix it" when, really, you know that the drawing is flawed and you should just start over.
If a drawing is rough but the pose and expression reads clearly, then it's a successful drawing. A clean drawing that doesn't read isn't worth all that much, like a car that looks great on the outside but has no engine or working parts.
A lot of people will probably argue with me, but I think Photoshop and the Cintiq has made this problem even more prevalent. The temptation is too great to add another layer to your drawing and draw it cleaner, or just start adding color and tones in an effort to try to "save" the drawing. But usually the basic pose is not pushed as far as it should be, or silhouetted enough, or has some other problem and needs to be re-thought and re-conceived. But that's a lot of work so instead we try to save the drawing that doesn't work.
I'm convinced that we use a different part of our brains to draw a rough drawing than we do to clean it up. It's much easier to be bold, draw through the forms and think "big picture" when you're drawing rough. Something about the cleaning up mode makes you get tentative and fussy if you're not careful (at least for me).
Part of the problem is that we don't get to see a lot of roughs. Almost all of the artwork we see in illustrations and comic books and animation is completely cleaned up and we never get to see the original roughs. So we don't get reminded very often that rough drawings are a viable way to work.
A lot of this "Kick in the Head" stuff bears repeating because it's tricky: it's the kind of stuff that we learn as students and we're very conscious of as we try to prepare a portfolio and try to get a job, right? But once you've gotten the job and you're in a professional environment, it's not really the kind of stuff that your co-workers talk about so there's never anything to remind you about it. And as human beings, we all tend to conserve energy as much as possible and cut corners when we can, so it's very easy to fall into the habit of being lazy and forgetting all this deceptively simple stuff. But then our work suffers, and over time we forget how to even do this stuff, and it can seriously undermine your work in the long run.
Obviously not every type of work can be rough. Most work needs to be cleaned up before it can really be finished (although I'd buy more comic books if they kept the drawings rough). But it's still important to go through the roughing out stage because so much great, loose thinking and discovery can happen in that stage. It always seems tempting to save time by just drawing your first pass cleanly but I think it can hamper your freedom and creativity as well as lead to stiff and restrained drawings.
I'm a big fan of Herge's Tintin books. A few years ago his final, unfinished album "Tintin and Alph-Art" was published. It contains several pages of his rough sketches...some fleshed out and some just a bare indication of what the final drawing would look like.
I wish we could see more rough work from great artists to serve as reminders of the value of drawing rough.