I fear you have the same counter-arguments that Tim Bunce suffered when he proposed DBI/DBD.
I know HTML::Mason, AxKit, HTML::Template extremly well. Eventhough they are extremly different i am aware that they really have a lot in common.
Once it is there it will convince. DBI was a new era. I just remember that many serious projects were founded by people who just wanna had fun. So it comes that they are often reluctant to learn a new API/Template engine. They have their preference and that is good so. Let them all use THEIR DBD::<template engine>.