Developer Tools (SSDT, BIDS, etc.)
An additional An additional option to ignore users not in source would be the best solution.
Allows DBAs to configure security for each environment and then reuse the same deployment script throughout the course of their migration process.
Faster Development, the developers do not need to worry about every user who may end up accessing the database in each environment.
Better support for CI builds and automated deployments, less concern that an automated process is going to wipe out user permissions that were added by the DBAs.