The pPHPlilb is not another templating system. There are so many out there, I really saw no reason to add another one.
It also is not an easy to learn system. The learning curve is pretty steep, I have to admit. But don' stop reading now (You still here?)! Once you mastered it - and really, it does not take all that long - you really can cut down in development! And besided that, you get a lot of security!
Another point of attention is: You probably should not use it for graphically heavy pages, thus where individual and creative layout is the main focus. The pPHPlib helps you to build applications with a common look and feel and does not really support "graphical creative" pages.
Also, if you are not using a database, the pPHPlib might be your wrong choice. The lib is built to especially abstract this layer and handles this for you.
Above does not say that you should not - or cannot - use the lib if above is true. Yes, you can, and if you are already using it for other projects, you probably should. It just is not used as its utmost efficiency.