I have a laptop running Fedora 16 with /tmp and /var/tmp as tmpfs (picked up the idea from Gentoo) with 16GB RAM and a netbook running fedora 16, which has 4GB RAM and never had any problems on both.
Why should applications keep tempfiles over boot? I personally hate session managers for example...
Why should the memory not being used for something useful on the laptop? Most of the time it only uses only like 1-2GB of memory, except if I use mencoder or something in /tmp.
With SSD in both, I have no swap partition/file either
My firefox saves temporary internet files to /tmp as well
So, I use /tmp for all the rubbish, the system is like a virgin after each reboot