I think the large UPS systems have something that most don't....they're being charged all the time, except when they're under heavy load, so there is little opportunity for one battery to run down the others.
And I think the "real life" answer to the original question here is that it should be fine to parallel the batteries we use on a robot, if they are disconnected from each other when the robot is turned off. If you could figure out a reliable way to do this, I doubt you'd have any problems. But I still don't see the need for running them in parallel on a T shirt cannon
