I posted you a long reply yesterday, but It's gone walkies, so I'll try again...
In short, No, you don't need a canbus plug as you can wire directly to the battery using the leads supplied in your Kit. I have done this on a 650 and it's worked perfectly.
I've also wired a garmin to my ADV using the canbus link - other than it being a little quicker to do, a bit slicker and more expensive

, it will switch the power off to your GPS automatically when the canbus goes to sleep. I imagine the only advantage being that if your battery on the bike is a bit iffy , there is less chance of it being drained overnight. Otherwise, it really seems to depend on whether you want to spend the money on the plug or not....