If you measure at the team level you get collaboration and much better solutions. It's a friendlier and more rewarding way to work with little turnover. It encourages product, dev and QA to work together and solve user problems in economical ways, not just throw solutions over the fence to the next stage.
And yeah sure I know if there's someone not performing up to their pay rate, you don't need numbers for that, you need to be involved and looking at their code.