@snapcase - I agree that the referral code spam gets pretty annoying. I wish there was a better way to deal with it (like the Market giving the ability to delete comments, although this could easily bring up censorship issues).
In reality, the referral system as a whole provides a great incentive for players to get their friends to play, and we get quite a few new players as a result. Unfortunately the spam is a problem as well as people posting exaggerated rewards for the referral codes is another problem.
Have you heard of any other games that have successful referral systems without producing all the spam? If so, could you give some details about what makes them different/better?