To me, the greatest contribution of mediocre scientists is that they teach their field to the next generation. To keep science going forward, you need enough people who understand the field to generate a sufficient probability of anyone putting together the pieces of the next major discovery. That's the sense in which the numbers game is more important than the genius factor.

Conversely, entire branches of knowledge can be lost if not enough people are working in the area to maintain a common ground of understanding.