Further, "what's in it for the employee" doesn't enter into it. I could easily say: famousactress is rated as 8.34 for her dramatic performances but only at 5.21 in comedic roles. That has nothing to do with your willing participation. I agree that it's depressing to be considered sub-par. But, there are so many people trying desperately to stand out that they're losing their way.
Google can't hire the right sort of people because every metric they have attempted to date has resulted in honest, dependable, clean-cut, recently-shaven, knowledgeable CS graduates from top universities. Sadly, they're trying to expand a business built (like so many other startups) by the ideas of college drop-outs (or similar), who were too busy doing something interesting to go to class and get good grades. Those interesting things aren't bragging rights (level 80 character in each class and each faction on WoW appears on very few resumes).
If I attach my HN information, you might disagree with my positions and opinions. I'd miss out on the job based upon personal bias -- maybe I should. But, you'll notice my karma here is _very_ low. I suppose that seems justified since you're not on board with this concept. But, is it? Reddit is commonly gamed. If I had an outrageous karma score, would you think highly of me or consider me a karma whore?
Well, you might say, you'd simply read my posts. Great! Now, consider you have a highly coveted position and 10,000 applicants. You can only read the posts of about 20 applicants. Do you take the top 20 highest scores? What if the 21st highest was actually the best fit? Well, then you blame your metric. The reason you blame your metric is because people will game their karma scores to reduce their competition to only 20 other people who were busily whoring karma and not building necessary job skills. At the same time, you can't deal with 10,000 resumes. You need a metric that's not easily gamed and that's as representative as possible of an individual's skills.
Just like I can tag you in a photo on Facebook without your consent, this site would allow me to rate your skills as an actress and as a programmer. My opinion of your acting skill is pretty much worthless (whether it's good or bad); my opinion of your programming skill is more significant -- but I'm not going to single-handedly make you or break you.