One really big advantage of the cadence sensor paired with the Garmin is it provides more accurate speed than the GPS. If you ride under tree cover or on very twisty roads, you'd be surprised at the wonky speed readings you can get when you use only the GPS. The Garmin defaults to reading speed through the GSC-10 if it's available and it provides more accurate data. Plus it's a must have if you're going to use your bike on rollers or a trainer.

As others have mentioned, it's not too bad size wise and when you're riding your bike you'll never see it. The only better option is the Trek DuoTrap or some crank based power meter.