For me, this is the primary argument. The vendor's installation is the vendor's. They will not consult you to see if your application will run under a future (or past) version that they want to move to, nor will they rush an upgrade just because your application 'needs' some feature. You are able to test, package, and deploy your application based on your requirements, instead of the current implementation selected by your OS vendor.*
* Yes, this is a broad brush, and there are some instances where using the OS vendor's version of perl is the RightThing, but that is a tradeoff and business / marketing decision, IMO. Just my $0.02.