Some do that. It is warm, but a full suit is much more cuddly looking too. I'd say the majority are full suits. You can see lots of photos of the fursuits (we had 215+ at the con) at http://pics.critter.net/view/ConPics/MFF_2006/ and there are now a bunch of video clips on youtube.com (search for "midwest furfest 2006" and "mff 2006" to find them.)

The weather in this area is cool enough by late November that wearing a full suit isn't too uncomfortable for the most part. I don't know how people do it in places like Florida and California, though.