SimpleT is about "reclaiming" PHP as a template language for rendering HTML.
SimpleT takes the view that PHP already is an excellent template language and when used purely for that purpose, is very easy for web page designers to work with (in particular because there are a number of tools popular design tools such as Dreamweaver with support for PHP syntax).
SimpleT takes advantage of PHP's output control functions to allow the correct seperation of Model from View (or Application Logic from Presentation Logic). Normally, developers who are using "pure" PHP for templates will do so by using functions like include() but, without output buffering, this impacts the ability to control when output is sent to the browser.
SimpleT also uses PEAR::Cache_Lite to allow output to be cached on the server side with an "expiry time", meaning exceptional performance can be acheived.
SimpleT also provides a lastModified() method to help with HTTP client side caching using conditional GETs.
As SimpleT is not inventing any (template) programming languages, templates are parsed by PHP itself, meaning it's also easier to debug, profile, etc. etc.
The one thing that SimpleT can't do for you, which other template engines that implement their own languages can, is restrict the templates to a limited subset of PHP functionality. Perhaps PHP's safe mode will eventually get the overhaul it needs to make that possible.