I almost always have my sweater tied around my waist at work when possible, and do it a lot outside of work too. Generally I do it because 1. I don't always have a bag large enough to fit the sweater. 2. I'm sort of cold but not cold enough to put the sweater -on- (this is mainly at work.. It warms me up a little still when it's around my waist). 3. More convenient access and I'm lazy.

Sometimes I kind of like how it looks that way as well, for some reason.