If you add a user in model then for every database created it will copy whatever is in the model database. One of my client want to assign user based on database name during database creation . He want to assign some user/role if the database comes from source 1 but want to assign different user and role if it comes from different source. If it is one or two database you can do it manually. We are talking over 5 databases every day. Database is attached, then analysis run on them and then dropped.

Model db will not able to do this for me.