I'm trying to combine two separate Role-based security tables into one. I'm doing ok so far, but I'm just wondering what the us_data column is for in the SFSUsers table?

No.  It will kill it and all of the users will no longer be able to log-in.  You are better off creating your own users preferences table rather than playing or trying to maintain this data. WE don't even do this in our medical software.  We have a user preferences table and we pull from this when the CurrentUserChanged event is fired.