Vladimir Zlatanov wrote:
The one place that the alternative engines will have to do real work will be that they should probably ship with their own versions of all the .tpl.php files that Drupal 6 will ship with.
I can see one problem with converting every nook and cranny theme_ function into .tpl and that is speed. It is related to the number of files loaded, to make matters worse, I doubt that any of the bytecode caches will help with that for heavy duty sites. That is unfortunate, because it is a rather elegant solution, fitted to how drupal works. Having said that, there are possibilities to improve it, depending what are we ready to trade. Caching - to db or a precompiled file full of theme_ functions are just two of the possibilities, you loose dynamism, but for production sites it is not required in the theme layer.
I completely agree. The primary thing to look at when converting a theme function to a template is how often this theme function gets used on a page. If it's a lot, then absolutely that function should not be converted. Some obvious candidates to NOT convert are theme_placeholder() and theme_username(). Making these functions into templates don't buy us much anyway. It's definitely a case of 'start with the big ones' and work to the small ones. Big ones are often used only occasionally per page; small ones might be used a lot.