Two circuit breaker switches controlling the same outlet, and a question about removing outlet tabs
Yes, there is a reason not to break the tabs on every receptacle that has four wires (2 black and 2 white). With the tabs in place the receptacle is a way of joining an incoming line from the panel to a cable which is feeding more receptacles and lights further on. If you would break the tabs on all receptacles, you would have to make these connections with lengths of wire and wire nuts. It would be a lot of trouble. 

The only reason to break these tabs is if you want the two receptacles in a duplex receptacle to be powered through two different breakers.

If you have a receptacle with one or both tabs broken and you want to restore the connection between the receptacles, there is a kind of pigtail with wire that can be used to connect the two receptacles, but it is trouble and takes up space.