The problem is that most of the individuals invited to a meeting are not needed, they know they're not needed, and they've got other things that they are needed

The problem is that most of the individuals invited to a meeting are not needed, they know they're not needed, and they've got other things that they are needed for. Agreed. Just because someone HAS to give their input in a conversation for an item to move forward, does not mean they need to be pulled away from their work for 1+ hours. Meetings really should be broken up into two roles: brainstormers, who are truly the people working their way through the item, and "expert witnesses" who can provide key details or contributions as necessary, through slack like you said.