Since most other distributions will have changed to tmpfs for /tmp in a year or so, their maintainers and users will shoulder the cost and burden of adapting ill-behaved applications (presumably those that assume that /tmp is limitless), so the Debian people could just prudently stay behind and switch over after all the work has been done for them. Or, they could give them a hand.
I'm personally an eager user of tmpfs for many things - the performance gains even compared against ext2 can be considerable. If anything, it could be useful to have a dynamic swap extension/shrinking mechanism, so that this would not have to be done manually. (I would not be surprised to learn that such a system already exists and is in daily use by just about everyone else except myself.)