There is a tradeoff between requiring the field and getting the user through the process. This is mostly a practical concern. Is anyone aware of a community/cultural rational?
The community rationale comes from a very old and entrenched part of Wikimedia: the idea that people have a right to complete and total anonymity, even when editing. It is also because the norm was set before the ubiquity of easy to use Web mail accounts (we predate Gmail by a few years, for instance), so users at the time had the idea that semi-anonymous email was not as ridiculously easy to set up and use.
To speak to the registration process: the tradeoff is almost not worth thinking about. Even among the users in testing who actually noticed email was optional, they still would fill it out much of the time. In the age of notifications and password recovery systems, users have a clear mental model of why email is helpful.