My dealer puts this on all bikes as well, as a courtesy on new bikes. You can buy just the dongle yourself for about $6-7 bucks at a lot of dealers without buying a charger. A lot of the dealers around here keep tons of these receptacles in stock.
The Battery Tender is definitely the way to go. Have the bungles, bungholes or whatever you called them on all my bikes. My brother has an '06 Gixxer 1K and still has the original battery. Keep 'em all plugged in between rides. Definitely a good idea.
I went out to install my battery tender dongle today and noticed that someone already put on on. I bought the bike brand new around 2 months ago and it only had 6 or 7 miles on it. Why would a dealer install just the dongle and not give the charger to the customer?
Dealers typically put those on so they don't have to access the battery compartment over and over while the bikes are in the showroom. I asked the same question of my dealer.
They install one mainly for maintenance purpose... so that they can plug it in to the battery tender while the bike is sitting on the showroom floor. As for leaving it there for the customer, it's an added service/convenience... as some of us already said.
It's cheap enough to purchase, but the hassle having to do it yourself... that's something I'm sure some of us don't mind not have to do.
Why don't they provide the battery tender/charger? That's an added cost. I'm sure they would if it can be added to the bike's price.