I guess I see Google search as this key entry point into the Web. By using backlinks as input for ranking results, Google is tapping into the power of social proof to organize this massive amount of information. When you cheat that system -- when you make deals for backlinks -- you're undermining the integrity of this system, and you're degrading the quality of people's interactions with it.
I might just be overly sensitive about this, but I really dislike when people cheat systems to gain an advantage.