Time-based releases are good for community
There was a bit of discussion lately on feature-based vs. time-based release schedules in OpenStack, and here are my thoughts on it. In a feature-based release cycle, you release when a given set of features is implemented, while in a time-based release cycle, you release at a predetermined date, with whatever is ready at the time.
Release early, release often
One the basic principles in open source (and agile) is to release early and release often. This allows fast iterations, which avoid the classic drawbacks of waterfall development. If you push that logic to the extreme, you can release at every commit: that is what continuous deployment is about. Continuous deployment is great for web services, where there is only one deployment of the software and it runs the latest version.
OpenStack projects actually provide builds (and packages) for every commit made to development trunk, but we don't call them releases. For software that has multiple deployers, having "releases" that combine a reasonable amount of new features and bugfixes is more appropriate. Hence the temptation of doing feature-based releases: release often, whenever the next significant feature is ready.
Frequent feature-based releases
The main argument of supporters of frequent feature-based releases is that time-based cycles are too long, so they delay the time it takes for a given feature to be available to the public. But time-based isn't about "a long time". It's about "a predetermined amount of time". You can make that "predetermined amount of time" as small as needed...
Supporters of feature-based releases say that time-based releases are good for distributions, since those have limited bearing on the release cycles of their individual subcomponents. I'd argue that time-based releases are always better, for anyone that wants to do open development in a community.
Time-based releases as a community enabler
If you work with a developer community rather than with a single-company development group, the project doesn't have full control over its developers, but just limited influence. Doing feature-based releases is therefore risky, since you have no idea how long it will take to have a given feature implemented. It's better to have frequent time-based releases (or milestones), that regularly delivers to a wider audience what happens to be implemented at a given, predetermined date.
If you work with an open source community rather than with a single-company product team, you want to help the different separate stakeholders to synchronize. Pre-announced release dates allow everyone (developers, testers, distributions, users, marketers, press...) to be on the same line, following the same cadence, responding to the same rhythm. It might be convenient for developers to release "whenever it makes sense", but the wider community benefits from having predictable release dates.
It's no wonder that most large open source development communities switched from feature-based releases to time-based releases: it's about the only way to "release early, release often" with a large community. And since we want the OpenStack community to be as open and as large as possible, we should definitely continue to do time-based releases, and to announce the schedule as early as we can.