You actually have to be really careful to design a language that's both useful and not Turing-complete. Regular expressions are one example of such a language, but they are not strong enough for templating.
Yeah. It wasn't actually intended to be Turing-complete, but then some excessively creative goddamn lunatic came up with a way to beat MediaWiki syntax with ParserFunctions into yielding a truly hideously inefficient "if" statement. Soooo they gave up and codified that so the CPUs wouldn't melt.
