It will wind up punishing his most prolific programmer. Odds are, one or two people might be the best employees who have worked on the most projects. If you have, in a 10-person department, one coder who is just a fountain of output and he's written 60% of the interface code, then 60% of the bugs will be in his code.

Explain that this system would make it look like the person who writes the most code is the worst programmer, and the person who writes the least code is the best programmer.