The last thing I want to do when hailing a cab for a 4 minutes drive is check ebay-style reviews. I just want a cab to appear in front of me when I step out of a building.
And for me, it's not even worth that. I just want a taxi that's insured.
I'm not against rating systems on top of what we have now. If it's worth it for you to wait a minute for a better cab, more power to you. (Hell, I might want to take some time to find a nice car if I were on a date or something.) But please don't take away the easy, efficient system we have now in most cities.
But they create incentives for good behavior and give those that have the time to plan information with which to make their decisions.