I think when the CLI importer was created, user management was still very unclear. Also it wasn't necessary yet.
But I think it would be quite useful for statistics etc - what would be your use case ?
There is a CLI command get avatars which creates a list of all users according to the #__activities table and then fetches their avatars. Maybe we can just rename this to get users and also fetch the user data along with the avatars.

what would be your use case

Agree. Why do we need to import them?

Later down the road as statistics are based more on the GitHub activity, it'll become more important to have the user data (at least usernames) so that it can all be linked together. If we ever implement a full profile, data such as commit count, activity points (similar to the dev site graphs), etc. would be things we want to display there or create badges that are associated with the user within the application. So at some point, we might want to consider having a user record for every user who has activity on the target repos.

Should we implement this?

later.