The problem is that GIT accepts a user.name of " " for some operations (for example when doing a simple "git commit"), but does require a "non-empty" user.name for others (like git commit --amend and git rebase). In case of the latter commands GIT fails with the message "fatal: empty ident name (for <email@address>) not allowed".
As people tend to do simple commits first, before amending or rebasing something, they may have to change their name after some dozen of commits which doesn't look nice.
This is certainly not a big issue, but it turns out to be quite annoying and I've already rewritten the history of a GIT repository once because of it, so that all my commits had the same author.
Proposed solution: GIT's requirements for user.name should not depend on the operation. Either user.name should be enforced to be non-empty everywhere or an empty user.name should be accepted everywhere. Perhaps filling out one of user.name and user.email could be sufficient.