Lazy/bad developers will always find ways to put business logic and presentation in places they don't belong. I like systems that -- rather than enforcing arbitrary and inevitably leaky constraints -- aim for maximum productivity and power in the hands of capable developers.
Let's take a real case: Smarty. To my eye, XHP is going to take a huge slice of the PHP template system market away from Smarty. The reason is that XHP gives you the native constructs of PHP (sans mind-bending hacks -- try referencing a class constant in Smarty) while staying at the right level of abstraction for fast development: chunks of valid XHTML.