The "Touch Screen to turn on" feature makes a small additional demand on the batteries. That's understandable, as the unit has to keep checking for touches to the screen, instead of just waiting for the user to push the on/off button.
The problem comes in when you have the "Touch Screen to turn on" feature enabled, and you have removed the main batteries, and you touch the screen. (I said it was rather obscure, eh?) In the initial release, the unit would try to turn on, putting all the load on the back up battery. If the backup battery was fresh enough and if you got the new main batteries installed quickly enough, no problem.
But if the backup battery was already low, then turning the unit on with only the backup battery in place might drain the backup battery and/or hang the unit, requiring a "cold boot" to reset things.
I assume the "fixed" units won't turn on when the main batteries are absent.
Steve