I seriously doubt that a financial incentive would change most maintainers' attitude towards pull requests. What is an appropriate piece rate for a PR review? There are plenty of highly paid developers who are doing a side project for free, for whom an hourly consulting rate <$100 would be well below their market rate. Well paid professionals doing a labor of love are not likely to be motivated by $5-10 here and there. Not all package maintainers are in this category, of course, but most are volunteers who don't expect financial compensation for it. Offering me $5 (or whatever nominal amount) to review a pull request is more likely to insult me than motivate me.

This is my problem with Gittip/Gratipay as well, the amounts are not enough to make a self-sustaining project, and even if it becomes a standard expectation, when someone else is willing to do it for free it would breed resentment. Open source should not be a service economy dependent on the gratuities of others.

Note that I'm not arguing that developers should work for free, just that small sums aren't likely to change their behavior. Sustainable business models for open source projects are difficult, and I suspect that tips are not often going to provide one.