There seems to be a frequent trend in the growing open source world to number releases as 0.x even if they are to be treated as a major release. In some cases, there may not even be compatibility between two minor releases. Can anyone help me understand if there's a good reason or rationale for doing this in certain cases, or is it just a perfectionist streak leading developers to feel like they aren't ever really done?
It's not entirely a matter of personal preference - in some settings (in the enterprise proprietary software world, at least) the decision of whether an update is a point release or something else triggers business and legal consequences, such as a requirement to provide the release for free (or not).
I suspect that paid support contracts for open source software deployed in business have similar consequences - what services are included and what have an extra charge, my depend on the level of the update.
For more general discussion about versioning here are two SO questions: This and this.