Quality assurance testing and CUTTING EDGE

[beejay41]

Or do you mean it doesnt read and write the user files but recreates them removing your changes?
Custom fields in user files is also new so you might need to use external data

Yes, the current multi-user plugin recreates the files and has its own auth. logic, as to who can edit users (doesn't alter login at all).
It's not going to play well with 3.4+

Edit:
Hooks settings-user and settings-user-extra are not supported by the multi-user plugin, so even now, it would brake any other plugin trying to add extra fields.