On the one hand, this flag will go away in later releases, and my comments are kind of nit-picking.
* We are not really compiling manifests, we parse them, and evaluate them as part of compiling a catalog (the manifests themselves are never "compiled")
* While it is correct that an expression like "1 + 2 +3" reduces "1+2" to "3" and applies "+3" to that, I don't think people really understands this. "Evaluation is what happens when you run / execute a puppet program/manifest". If you like, it could be expressed as - The future evaluator is an interpreter of the parsed logic. 

Contrast: We may later write a different evaluator that first compiles the instructions to machine code.