To come to the point immediately: it is all about bad press. I understand that release schedules cannot always be kept and I think it is better to postpone a release than to enforce it in a broken state. It is an entirely emotional thing. Being enthusiastic about the release in April ("it is out this week"), it is disappointing to still wait for it about three months later. My statistics on reporting bugs about X.org and related software has significantely dropped, because I am pretty disillusioned after waiting for a long time for the new releases of Mesa and X. Yes, the projects do not need my bug reports. But it is the whole "bad press" thing that hurts the project.