I have only one 12-volt accessory outlet in my little Honda Fit. And my Garmin GPS needs one pretty much full-time. It doesn't run very long on it's battery. I don't think it was designed to.
So the GPS ties up the only available outlet. Which is a pain when I need to plug in a mobile device for charging (like a smart phone or tablet).
To solve the situation, I wired up a female cig-lighter style plug to the fusebox using a fuse tap. I then ran the GPS power cord (with it's male cig-lighter plug) under the dash and have it plugged in where it's all out of sight. That frees up my one accessory outlet for various charging needs as they occur. Yay!