My 08 Electraglide Classic came with the plug-in already installed from the dealer. It hangs down from under the seat area on the left side next to the top of the rear cylinder.
I use it for plugging in my electric vest when riding and for plugging in the battery tender when parked. I'm pretty sure most new bikes up here come with the plug-in installed from the factory so maybe it is standard on Canadian models.
I've never used a batter tender before but figured with all the modern electronics on these new fuel injected bikes along with the stereo and alarm system, its probably a good investment.
If your bike doesn't have a plug-in connector already, it's only about a ten minute job to install one with a few basic tools.