Whirlygig,
It is not a particularly good indicator of how good a pilot you are or will be
I have to disagree, based on my own experience, I don't think that I was anything like properly trained after minimum hours. I didn't had any "mishaps" so I suppose that I was trained after a fashion but I needed a lot more dual especially in the instrument flying department since I very nearly had a nasty end flying into inadvertent IMC conditions shortly after obtaining my PPL. I believe that incident would have been less likely to have occurred if I was better trained both in practical flying and by way of ground school to be able to identify the conditions to avoid.
It is my belief that a lot of PPL's do not continue to fly once they have their license simply because they cannot justify flying around their home airport like a fly around a jam pot and are somewhat nervous of longer excursions which pose threats of encountering bad weather and getting lost etc. On the other hand, longer training would be likely to put a lot of people off of training in the first place on cost grounds. So, the choice is to put a post solo student in the air with a license and leave him/her to learn by experience or ensure that they have additional training under supervision and give them the license after their instructor/examiner feels that they are adequately trained. Just my opinion.