An advantage of the Rebel 250 as far as never leaving you stranded, the battery / headlight / starter can be non-functional but the bike will still push start and run just fine. I have actually used this when my battery popped a cell (I have not bought a BikeMaster battery since BTW) and was stone cold dead, none of the accessories worked but the engine ran fine with a push start (which is super easy with this bike), and I was able to get to a garage anyway. Don't forget to use hand signals if forced to do this, the turn signals and brake light are not working either in this state. Once I got to a shop and installed the new battery, everything was back to normal.
I've indeed experienced that in some usage patterns (i.e. slower speeds, short trips) the battery 'input' is exceeded by the 'output' used to start the bike. It takes time for the generator to recharge the battery, it will do it but if your ratio of starts to run time is skewed towards the former, there will be a net loss that will need rectified by external charging. This is true of any bike, generally anyone who needs a tender needs more regular long trips in their riding lives.
I've never needed to use a tender on any of my bikes as long as I am actively riding them, but that's easy for me since I live in the sticks and nothing is close.
If going the tender route, don't go cheap. A couple dollar el-cheapo 'trickle charger' will murder an expensive battery, get something with built-in intelligence and it needs to support AGM batteries properly if that is what is installed, a non-AGM tender will cook an expensive AGM battery to death.
2008 Rebel 250, 2014 Street Triple, 2012 Voyager 1700, 2014 FLD Switchback, 2017 Guzzi V9, 2017 Concours 14