I use restrictions when I am trying to do a certain theme to my games, not just throw a wrench in someone's plans for their snowflake.
For example: At the moment, I am working on a Dresden Files type of game where I have the Wardens and I have the Council. I have restricted that in order to be a multiclass Fighter/Spellcaster, Magus, etc you have to be a Warden which entails it's own set of prereqs. In order to be a member of the council you cannot cast spells of the Necromancy school.
This is just a taste of the type of game I am planning. Now I know that it won't be for everyone but the option to play it will be there for those that do want to play it.