A person can pick up flying experience with a kit-built trainer just as well as an ARF. If the fellow is going to be designing a plane from scratch, it'll give him good insights into the construction methods and design requirements for an R/C plane. There's also the knowledge of basic building that can't be gained with an ARF.

Is the goal to learn to fly RC, or to successfully design, build, and fly a scratch built model? I'd say it's the latter, wouldn't you? Sometimes we seem to forget that different people have different goals, and our advice should be tailored to their individual goals.

It's very easy to over or under engineer a model airplane if you don't have any experience in how they go together. Valuable insights can be found in the experience of building a kit. That experience can save a lot of heartache and re-doing in the future. There's not much similarity between modern full-scale aircraft design and RC. The materials and methods are quite different.