It will simply lead to people gaming the system.
When analyzing developer performance, it would be best to use statistics as an additional source of information. As such, statistics are extremely valuable. Charts and graphs, measures of confidence, trends, correlations – all great stuff.
Statistics cannot be the sole measure of the team’s performance. If it is, people will start working to satisfy the statistic instead of creating business value.
There are many examples of statistics that people have used in the past. Some people have looked at the lines of code created. Then people started adding more inane comments, newlines, unnecessary tests, and dummy code.
Another idea was to look at the number of commits. But then, people started making more and more smaller commits, even of things that aren’t any actual improvement.
As Scrum gained in popularity, people tried tracking the number of story points. Companies hired scrummasters to make the team more Agile and increase Velocity. But then the content of a story point was reduced, giving the appearance of improvement, even if it was all an illusion.
Someone else watched Office Space and decided that hours spent in the office would be a good metric. But then people spent half their days surfing the web, reading Facebook, and looking for a new job.
In each case, depending solely on a statistic led to the abuse of the statistic.
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