The issue is momentum: with HN/Reddit's time-based ranking algorithms, the first few upvotes are extremely critical. On Hacker News for example, getting ~5 upvotes in the first hour for a otherwise-unexciting product will usually have it touch the front page, and that could be hit with just little collusion.

Getting the rest of the upvotes and the traction to hit the top of the front page, in fairness, is dependent on non-collusive votes. The issue is that subtle collusion may get products exposure when they otherwise wouldn't in a fair system. (although, that's the startup world in a nutshell. I'm not fond of it.)