My English professor once said that how clearly a person can communicate to others through his writing reflects greatly on his thought process. Poor writing skills are pretty much due to messy thought process.
As a math teacher, I see something very similar. I really try to stress to observe how I do work on the chalk board, how I organize the problem, and how each step automatically leads to the next. Much of the math I teach is mechanical in nature -- there really isn't a whole lot of thinking involved -- just do the steps in the right order, and you'll get there.
When I look at homework and exams, I see a very high correlation between how neat and organized the work is on the paper, and how well they really end up understanding the material. The very neat, organized ones end up being the ones who understand the material and get the answer correct, and the ones who have bits and pieces of correct work scattered here and there in a disorganized fashion quite often get the final answer wrong.
Another interesting thing that I have observed: usually the neat and organized ones have very neat and clear handwriting, and usually the ones who can't seem to organize their work have handwriting that runs from not very neat to illegible. I wonder if there is a correlation between handwriting and clarity of thought?
"The guilty one is not he who commits the sin, but the one who causes the darkness."
Clearly, he had his own strange way of judging things. I suspect that he acquired it from the Gospels. -- Victor Hugo