Because /tmp is a world read+write+execute directory it can be advisable to have it on its own partition and to mount it as nonexec. This means should someone be able to get a malicious program into your /tmp directory they will have a harder time executing it.
Also by having /tmp on its own partition if something goes crazy, from malice or mistake, with writing to it your entire drive won't get filled.
Some information on this at
http://www.debian-administration.org/articles/57I know you are running CentOS but same things apply.