This fits on those, "on paper sounds like a good idea" but in reality I cant quite see it.

1. easy to game 2. suffers from the lines of code problem, is someone a good developer because they spend 99% of their time inside emacs

I used rescuetime for a while and I didnt see a particular correlation between how productive I felt one day/week was and how productive rescuetime said I was.

I feel that github has really nailed this public profile of developers that accurately shows what they would be like to work with. I have hired and been hired in the last year and both of them came down to github / open source.