I've been using Fuel for a number of weeks now, and I'm generally really liking it. I agree with the Twig recommendation though, and did drop Twig in almost immediately.

To answer your question specifically: it would be smart to use Twig by default, rather than developing your own custom Fuel template engine, because you would need to spend a significant amount of time to get something with the same feature-set, build quality and documentation coverage. Would this time not be better spent elsewhere?

As a pretty content Fuel user, I should also mention that your tone doesn't benefit the project.