XSLT is wildly more than a templating engine. It can (and has) been used to e.g. specify a protocol and generate software based on it. See XCB for an example. With a sufficiently large corpus you can run queries on XML and generate arbitrary media.

As with most overbearingly flexible technology, it's an incredible pain in the ass to use efficiently, and XSLT processors tend to be plagued with complexity and concomitant performance problems.