Fired Up,
It sounds like this "Battery Tender" is a fully automatic trickle charger switching on and off to keep the battery fully charged, like on an emergency crash cart at a hospital on a sealed battery. If it is kept in this fully charged state, you are going to be in good shape leaving from home, and can quite conceivably get a couple extra years out of it. To test it, try leaving it unplugged for a week to ten days and see if it drops off dramatically. A lot of batteries will start if kept on a "tender" but when they are aged they will not "Hold" their charge as they get older. It would progress to the point where in the aged state it would still start after hours down town, but overnight be down enough to be having problems, and a few days would not start the the HD.
If you were always leaving it on a full maintenance charge, you are right and you might get a couple of extra years out of it, alway starting there from home on the tender like your fully maintained fire trucks ready to go. But a trip away off the tender, and it could be losing 1% of its charge per hour or more... Where a new battery might only lose 1% every couple of days and be OK for a couple of months and still start. You get what I mean? Start there from home OK, but on a trip it might let you down not plugged in..
Spydy: You are right on excessive heat, called "heat degradation" and that shortens battery life as well with the wrong kind of charger. It has to shut down or taper off the charge to nil, or the battery has to dissipate the energy you are pumping into it, by giving it off in heat. That kind of wrong charging will also dramatically shorten battery life. I hope this is a help. We go through alomst a million a year in solar systems and R&D on this kind of stuff....
We have to be careful about these things in the Outback, and depend on solar more for that kind of maintenance. If I had to stretch a battery life on a Tourer for some reason, my "battery tender" would probably be a Tour Pack rack mounted small solar panel and solar regulator to do just this same function keeping the battery tickled up if I was away on the road. Hope this is a help.. Cowboy in Oz...