Wow, fascinating question. I think that the people who play the characters in MITM really make the show perfect, and it wouldn't be what it is with anyone else playing each part.
The harmony that the actors are in in real life contributes to their convincing--and often heartfelt--performances on television.
I think that you have to keep the lives of the actors and the characters separate, because they are two different people and one of them doesn't really exist. But the thing is--the characters DO exist in our heads and in our imaginations--and they exist in those places for the people who play them too! They show some similarities because the actors' ideas of the characters are influenced by their own ideas. How well the actors get along influences how well their characters get along. And fictional or not, the Wilkersons are not dysfunctional and represent something that many families can strive to be.