if you have speakers in the rear deck and a sub in the trunk, you will distort and likely damage the rears. putting them in enclosures is a way to prevent that. rear door speakers don't suffer from the pressure of a trunk sub, so they are safe, and in better locations for rear passengers. one speaker blaring behind your head isn't very enjoyable.
true "rear fill" is intended to simulate the room acoustics of a larger space - which is why it's delayed and filtered. full range speakers in the rear is not "rear fill".
there is nothing wrong with wanting to be surrounded by speakers in a car - for some it's part of the experience. granted, more speakers means more time arrivals which typically results in reduced sound quality. and if the rear speakers are of lesser quality, they can only hurt overall performance.
but rear passengers like to have some music, and kids do not need bass. when kids are in the back seat you should be turning your sub(s) off or very low. turning on rear speakers can be as simple as adjusting a fader, or flipping a switch if they are controlled with relays.
i recommend installing rear speakers in the rear doors for your rear passengers, and having the ability to turn them off. if in the rear deck, get an enclosure around them (difficult with some torsion bars).