I sometimes use a pattern and sometimes don't. Probably moe often than not I don't use a pattern...but I'm rebelious and even when I have a pattern I often change it into something else.
The only real plus for me in using a pattern is that all your cutting is laid out for you. Sometimes I like how that can save you time and from having to bust out your math skills. (Math is not my strongpoint...and I've vowed to make no efforts to improve that. Lol*) I also rarely over buy fabric if I'm using a pattern. When I go my own route I end up buying hoards of fabric, and fudging the math a few times resulting in many extra orphaned blocks. But I still like to make up quilts as I go. ;) Sometimes with the aid of EQ which helps on the fabric hoarding aspect. Although I wonder if subconciously I'm just enjoying buying all that extra cotton... ;)