If you want to keep your accessory on when you are on the side of the trail such as if you were using a GPS, you will want to wire it to the battery with an inline fuse.
Also wiring it to the factory plug-in, limits you to 3 amp fuse (if using the stock fuse). I blew this fuse a bunch of times running my heated shield not because the shield was defective, but if you get a little snow on your plug and then connect it, it pops the fuse.... pissed me off