Do you remember how perl 5.6.0 and mod_perl worked together? I use HTML::Mason, and HTML::Mason and perl 5.6.0 do not cooperate very well. In fact, you get (near) random segfaults.

That is a specific example for why upgrading platform software can seen as rather risky. Upgrading will mean that you need to develop contingency plans etc etc. It's a hassle, and if it works, why break it? (Yes - I know. Security, security, mindshare)