Another very valid reason for a run and break which I haven't seen addressed here (if it has been, I missed that post) has to do with situational awareness in a formation. In a close formation, the lead pilot knows where the formation is in space and where the airfield and other traffic is, etc. The other pilots know where Lead is and that's about it. They may have periferal awareness of their location over the ground, but may not have total awareness of the active runway, where they are in relation to it as they approach, other aircraft in the circuit or approaching.
By leading the formation to disassemble via a run and break, Lead hands the pilots off to their own navigation from a known point - over the runway. They therefore know excatly where they are going to be when Lead calls for the break and they know what they need to do to get on the ground safely - because it's the same every time (left or right traffic not withstanding).
To try to break up a formation otherwise, especially if the flight doesn't go off exactly according to the brief, would probably require esoteric radio-hogging descriptions of angles and distances to the runway and a pretty loose and wooly seperation of the aircraft in the vicinity of the airfield. I was forced to do that at an airfield when someone not associated with the formation I was leading decided he knew our situation better than I and denied us a run and break. It took 10 minutes of maneuvering and radio calls to get the other guys to the airport and properly sequenced for landing. And then he bitched me out for all the chatter on the radio.
Having done it plenty of times both ways, I much prefer a run and break because I know where I'll be when I break from the formation. I've done the "seperation away from the field" bit and couldn't actually find the runway until I was on final behind the preceeding aircraft - not great for establishing complete awareness of the airport and other traffic.
Just another thread to this thread's bow!
Pitts2112