And you can't simply throw the Twig classes in app/classes why? Oh ya...no reason. Why should we lock people into a specific templating system? We are working on something for v1.1, but it will still not be core...it will be a package.
You could add Twig into Fuel in the time it took you to write that comment.
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.
Sure, and you could provide zero abstractions to the database (with an ORM) and tell people to grab their own library. For one of the core pieces of MVC, I would hope that you'd actually develop into the V part more.
Your replies to any not purely positive comments or criticism in this thread or elsewhere come off as very abrasive and rude by the way: not the kind of attitude HN expects and is certainly not the way to attract more of a community.
Your framework looks very neat and I'm interested in seeing what it turns into in the coming months/years: don't spoil your reputation before you even leave the gate.
You could add Twig into Fuel in the time it took you to write that comment.