True, it isn't necessary to keep the battery on the BT continuously. You could connect the BT and when it goes to solid green disconnect it, then after a week or two do it again, etc.. It's just more convenient to keep it connected versus remembering to go out every X number of days and connect the charger, then remember to go back out and disconnect it. That's really what a "smart" charger is all about; taking something many people would forget and automating it. Btw, it makes a difference also if you leave the battery in the bike versus having the battery sitting in your basement. Today's vehicles always have a current draw when the battery is installed, thus the need to either ride them often or charge the battery when unable to ride. Take the battery out and put it in a cool dry place, and the self discharge rate of the AGM battery is very low. If you started with a fully charged battery in good condition and just let it sit out of the bike, it would probably be just fine for several months. Once again, it's all about convenience and how much you trust your memory to check the state of charge on a regular basis and fire up the charger when needed.
Jerry