For the most part I'm talking about issues those projects already have, such as being hostile to new users or contributors, and especially to various specific groups. As for "why": much like "Free Software" versus "Open Source", there are ideological reasons and practical reasons. A sample of practical ("Open Source"-style) reasons: because you want more users and contributors, because you want contributors who don't all think like you do, because less energy spent dealing with awful people is more energy spent hacking, and because communities that people have more fun in are communities that grow much faster. Additional ideological ("Free Software"-style) reasons, which apply whether the rest do or not: because being decent to everyone is not too much to ask, and shouldn't have to be asked for in the first place.