The negative aspect is that in games where stats are viewable for all, they become a badge of honor/mark of shame, and you'll see logic like "you only have a X rating, so you need to L2P before your opinion matters". Which may or may not be valid logic, but it definitely promotes a level of animosity which is detrimental to the community.
The positive aspect is that realistically, things like win/loss are good measures of player effectiveness (with large enough sample size), and they can be used to track whether the things you're trying are working or not, or if you're playing the role that's best for you, etc. Also if its viewable in-game before the match like in some other games, you can decide what you should do that would be best for the team (example, if some guy is way higher rated than you, maybe you should support him as best you can, whereas if some guy is clearly a suicidal fool, then you don't want to be following him around).
Edit: Added the option for people who don't care either way.
Edited by Broceratops, 20 July 2012 - 09:37 AM.