IIRC the only real argument in favor of non-machined rims refers only to non-machined rims INTENDED to be used with brakes. I believe it goes like this: Creating an even braking surface from the get-go is harder than machining one on later. The quality needs to be higher because the tolerances you're working with are tighter.

I don't necessarily agree with any of this, and I don't think it applies here even if it is true.