Until your OS patch changes the OS's version of perl (or a library) under the hood, breaking the assumptions made by your application. This happens in large enough organizations where 'the canonical OS platform is X.Y.Z, and all machines must be upgraded to X.Y.Z by this and such date' (yes, it does happen :-/ ). Using your own perl gives one fewer moving part not under your control.

If you are releasing on CPAN, then yes - test far and wide. There are times for both approaches.