I think people overtake when they can. I can't think of an occasion when somebody has refused to overtake because it had no "reward". Proffessional drivers are mad: they not only take enormous risks, they also have an ego the size of a transatlantic.
There are plenty of series with more overtakes, like stock cars (DTM, V8, NASCAR) you can watch.
F1 is a resources race and an engineering competition more than a competition of drivers: most of them are as good as it gets.
I'll update this year's figures soon. Meanwhile check this:
I see a strong correlation between the strength of the car and the number of overtakings. Notice that Massa (this figures are for 2007, sorry, I know figures for 2008 are interred somewhere in the forum) has 20 overtakes in the whole year. That's slightly over 1 overtake per race...
Notice Schumacher figures.
Conclusion:
the proposed system would be exploited by teams, they're nobody's fool. At least (I'm latino, sorry
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) I would qualify in a position well below my car possibilities, just to earn the points you would distribute. How would you cope with that?