Perhaps you are right, perhaps the purpose of a competitive jam is to quantify the quality of the work and the skills of the developers involved, but also, as you said, it's not really that helpful, head-hunters look for specific skills, not winners, same for publishers, they look for games that have a community around them, winners might have the vote of the people, but not necessarily their full interest.
I'm not saying competitions are bad, not saying they aren't fun, they are for certain people, but having that mentality all the time can be detrimental, I've seen people that are so focused on winning that they forget to build a network, and having a huge network will open up way more opportunities than winning jams.
[Edit]
I'm not implying that you should never enter a jam with the sole purpose of winning. What I'm trying to say is that your mind should align with your goals, if your goal is to win game jams then, by all means, do that, but if your goal is something else then consider what kind of mentality would benefit your goal the most.