I always mount /tmp as tmpfs, but I have large RAM and know exactly what I am doing. I used to analysis ~10G of data, and reading from RAM was at least 300% faster, even including the heavy data processing. I also rendered movies using tmpfs when size fits, and again observed dramatical difference.
The problem is: if a user cares about that performance difference, he probably knows how to use tmpfs himself. Setting /tmp to tmpfs will confuse normal users when an application fails. Given the popularity of those big distros, it might not be a good move. Even Firefox doesn't store tmp file in /tmp unless you override it in about:config. It might be worthwhile to check how existing applications are using tmpfs (/dev/shm). I have a feeling that most applications don't care at all.