Here's a use I made of it in relation to Lists. Registration is very basic with me, so many fields are not filled in at the start.
When a User registers they go into a 'pending usergroup' through auto action then when profile is completely updated they are moved to the 'main usergroup' manually in user manager. Special view and public view respectively applied. (as you don't want scrappy bits showing up in Lists viewed by Users).
I thought about this solution for a while, so could save you time. Careful though Joomla goes weird with >70 user groups.
nant
