Good point. Any benchmark can be co-opted, that's why it's useful to have multiple benchmarks. There's a Dilbert that tells the story on it well. [1] This is true for a lot of fields. If you over-optimize on individual player scoring in basketball, your team might play worse even if there is a correlation between individual scoring and team scoring.

[1] http://search.dilbert.com/comic/10%20Dollars%20Bug%20Fix