The case isn't "my kid mellows out on weed" it's "my kid has behavioral problems that prevent him from functioning at home & in school that are alleviated by weed".
What is the advantage of this over raw plant material?
To a point, consistency. It might not even be pill form: vaporizers, inhalers, and other such things might work as well. It might be that the best way to do it is via hash instead of the unprocessed plant.
All that said, there is some concern with the synthetic. I have a feeling the very reason the plant works better is because of the high. For example, MS patients have problems with muscles. Patients report better mobility and less pain, yet the muscles don't actually improve. It seems it just doesn't bother them - and I imagine some of the synthetics wouldn't produce this effect.
It doesn't mean that synthetics won't work for some folks, though, and might be better for kids.
Consistency.
However, if consistent effect is the goal, I want to suggest that oral administration is not ideal in the first place, and that oral administration of isolated chemicals even less so.
Achieving the latter is more accessible via slow, careful titration via vaporization or smoking of high-quality plant matter.
This is the case at least for me and people I know, including several patients with otherwise debilitating illnesses.
Additionally, I think that the ability to garden and grow medicines (not just cannabis, but plant medicines generally) is a great way for patients to gain some control (or at least the sense of control) over their illness. It also is often more economically viable as well.
https://www.google.com/search?q=marinol&ie=utf-8&oe=utf-8
etc.