I think system synergy can be optimal or excellent. Same same throughout is definitely one way to reamin close to it without taking much chance of diminishing it. Obstacles do stand in the way of that 'all the same name" path ocasionally. Mostly money, as I see it. Philosophy is another. An approach to a system building exercise can demand changes.
At some point in a good number of systems departures from having all the same are dictated, and no longer a choice. Just now I can't think of one manufacturer that makes everything a high end audio, or video system requires, 'soup to nuts'.
Mostly though, I think the departure from an all "xyz" system is fueled by the desire to obtain a better level here and there throughout the system.
In fact I think many times system synergy is bypassed or overlooked as the newly acquired device is not thoroughly optimized and integrated into the system. it's simply plugged into an exsisting system and followed by only a cursory decision on it's 'synergy' with the balance of the equipment on hand... but such is the way of things.
I would believe many great opportunities are passed in just this way. there's more to this than just components. tha'ts the "art" part of system building. It takes more than getting, hearing, and then getting another one. it takes time too. Oh, yeah, and money.