When I schedule in work, usually it involves a client paying me for that work. One of my clients for one of my larger ongoing projects expects my team to produce 20 days of development a month. How exactly do I explain to them that instead we will be doing 18 days of development for them, and two days contributing to, say, Rails? I don't think they'll really see that two days of contribution to the improvement of Rails is equivalent to two days of adding features to their product.