It could work, but I think it would only aid those at the top of the races to get more points. When you're controlling the race from the front, it's very easy to save your kers for one fast lap at the end, thus enabling you the fastest lap.
For the lower leagues, who have teams without kers, the fastest car will get the fastest lap, and it would only spread the field out further in terms of points, if only by a little bit.