Lets say you want to validate the room.people collection and prevent the user from adding more people to the room than will fit. If they do try to add more people than will fit, you want to display a nice error message on the page and let them try again...

This isn't possible using the standard has_and_belongs_to_many due to these two problems:

1. When I do the assignment to my collection (room.people = whatever), it immediately saves it in my join table (people_rooms) rather than waiting until I call room.save.

2. You can "validate" using habtm's :before_add option ... but it any errors added there end up being ignored/lost. The only way to abort the save from a before_add seems to be to raise an exception... 

We don't want to raise an exception when the user violates my validation. We want validation of the people collection to be handled the same as any other field in the Room model: We want it to simply add an error to the Room model's error array which we can than display on the form with the other input errors.

has_and_belongs_to_many_with_deferred_save solves this problem by overriding the setter method for your collection (people=), causing it to store the new members in a temporary variable (unsaved_people rather than saving it immediately.