Experience has shown, that quality is one of the first things being cut in operations/production environments when being time constrained, so preventing one source of constant work improves quality and security instantly and irrevocably.

Pre-built templates, especially those which can be downloaded from the internet, are of doubtful trustworthiness. It's trivial to open backdoors, install keyloggers or run DDoS clients if you have full control of the binaries which are going to be run in a VE.