A good question from the Savant-Talk list today. The quote is from the questioner, and the remainder is my response (almost verbatim from the list).
But could You PLS explain me shortly the main idea of plugins? I mean I can not understand that there is a need to create for example any html form fields by php... I still see the idea of templates as separation of business logic from display logic. From my point of view such example form fields shoud be still in template but not generated by php. So maybe such ones should be provided quite so as a templates or something like that. Only the values should come to them.
Well my wonders goes farther - why to use ANY php or savant function INSIDE template...(?)
First off, use of plugins is not required, of course, so if you don't like them you don't need to use them. This is kind of a weak argument, I know, because the code for plugin support is there whether you like it or not -- in Savant3 this will end up being about 30 lines (with comments it's about 50 lines), which I think is very small for the power you can get from them.
But second, and more importantly, they are convenient time-savers. I completely agree with you when you say you "see the idea of templates as separation of business logic from display logic." To that, I say: a plugin is just an encapsulated, reusable portion of display logic. It gets tiresome to write forms by hand all the time. Instead of *having* to write a select box by looping through options passed in as an assigned variable, and mixing the PHP echo code with the literal HTML, you can use one line in the template to call the 'form' plugin and generate a select box with correct items pre-selected for you.
One argument against plugins is that they are not a necessary component of a template engine; if you want convenience methods, you can instantiate your own output objects and assign them to the template, or write them as global functions, or call them as static class methods. This is a perfectly valid argument. However, to my mind, that kind of thing is a "pattern" of development, and will end up looking nearly the same across all implementations. All that Savant does is take the common aspects of that pattern and codify them for common use so that different developers can collaborate easily and effectively using the same higher-level tool set.
This is not a comprehensive defense of plugins, but I hope this brief essay begins to show why plugins can be useful and worthwhile in a template system.