If a battery decides to give up the ghost, it can do it all by itself and a on/off switch wont make any difference, It has happened to me before. Internal breakdown, melted, smoked and then went to Davy Jones's locker.
As far as I'm concerned, the on/off will just save your atty from being nuked if the push button is held on while in a pocket or purse. I think after its been held on for 10-20 seconds its going to burn out the atty coil anyway and that will shut it down just like opening a switch. It's a unintentional safety switch.
I guess more than one maker is using the plastic. Personally I don't leave batts in my 