We wanted a templating engine that was deliberately simpler to have a cleaner separation between the logic and presentation of a site. Other templating engine such as smarty puts too many smarts into the templating engine.

I suppose it would be possible to do so on a site-by-site basis... You would need to replace the defaultAction() method of your Page_Controller to make the appropriate smarty calls.

The other thing that you would need to do is get smarty to make the appropriate calls to the ViewableData methods obj() and XML_val().

What did you see as being the advantage of using smarty?

Documentation for the templating engine is here: http://doc.silverstripe.com/doku.php?id=templates

It may seem short but there's not that much to the templating language.

The bulk of the complexity is all the methods that are called on the controller / data object classes. The API documentation is helpful there: http://doc.silverstripe.com/assets/classes/