The big trouble for me would be "Everyone must now be careful never to store a file in /tmp that might grow large". I often use /tmp for all kinds of temporary storage. Be it a small text file or a GB big tar file.
I remember CD burning softwares and torrent clients asking for special temp directories to be used for that.
Allowing every day applications to create GBs of tmp files would mean you would need to worry once your partition has less than 100 GB free.
However, nobody stops you from mounting /tmp anywhere else or even disable tmpfs.