Using non-PHP templates with the excuse of separating logic is nonsense. If the developer doesn't understand what the business-view logic separation is and how it should be done, then the problem must be addressed appropriately. Otherwise you end up with HTML in business logic or business logic in templates -- no templating engine is going to save you. You have to teach the developer the basics.
And if the developer does understand that, the templating system is only a limitation. It doesn't add any value to the development process, only overhead of learning a new syntax, keeping another library up to date, and slower execution. While the latter can be solved with caching and whatnot, this only remedies a problem that otherwise wouldn't exist. So, templating systems offer no value, no advantage at all.
There is one exception, though, where I think using a non-PHP templating system is reasonable: when view-logic programmers must have limited access to the templates. For example if you're a provider for a blog-hosting system and you want to allow your users to personalize and code their templates, without allowing them to execute arbitrary code. This argument, however, does not apply to cases where a designer is willing to learn a little code to help programming the UI. If he can learn Smarty, he can surely learn PHP.
