Developer A works on Project X. Everyone loves Project X, it becomes very popular, and consequently, Developer A receives a bunch of Gittips to continue work on it. Say this amount is nothing to sneeze at...$250/week or something.
Developer B thinks Project X is good, but it could be really great. Developer B really loves to make great software, and he doesn't care about being paid for his open source contributions, so he decides to fork it into Project Y. Project Y has significant improvements and changes, so that if it could be compared objectively, it would be better than Project X across the board.
Developer A sees will see Project Y as a competitor and a threat to his tips. Now he has a financial incentive to discourage the adoption of Project Y. He doesn't want his tips to disappear, so why would he bring attention to something better? Some changes and improvements could be ported from Project Y to Project X, but if the overall direction of Y is different, there would be no way to reconcile the disparity. Project Y would continue to gain adopters, and Project X would lose them.
TLDR; Financial incentive to discourage competing OS projects is bad for everyone.
I've seen this happen before on OS projects when there wasn't money involved (only pride and reputation), so I think this problem would be intensified by cash. It would be cool if this never happens though.