Hi Arnor!
What about security concerns?? You'd be on the mercy of another projects' bugs, continuity and activity. And constantly trying to be up to date to some other people's codebase. Just thaughts...
Yes, this is true but applies to any dependency one has in a project. However the Smarty project is deployed on many, many sites. Also the ZEND Framework endorses its use, so the chances are that Smarty will be well-maintained for quite some time in the future.
Best regards, Garvin