really? it's not THAT big...

i also got an edge 500 recently and i do have the sensor on my bike. before that, i had a cateye double wireless, and the sensor on that was at least as big as the garmin. it's not like it gets in the way, throws your bike off balance, or anything...it's just a little sensor. might as well use it if you have it. cadence doesn't really influence my workouts much, but it's nice to know sometimes.

edit: also, the speed and distance from the sensor are more accurate than from the GPS (assuming your wheel size is entered correctly). so throw a magnet on the wheel and use it for that, too.