I've often found that by TeXing my notes, I spend much less time writing because I type a lot faster than I write. As a result, I'm able to pay much better attention to what the teacher is saying rather than spend my time trying to write it all down.
In general, I've found that if I attend a math class and don't write anything, I learn the material much better, since I'm actually spending the time in class absorbing the material rather than writing it down. The only reason that I do write it down is because it allows me to go back over what happened in class. Whereas when I don't write anything down, I may learn it fairly well the first time through, but that doesn't mean I'm able to learn it perfectly, hence I do have to go over it at some point.