While I understand and appreciate that some viewers may be bored by a lack of ethnic characters on a particular show, I'm not among them. If the stories and acting intrigue me (not always the case with Brothers and Sisters), I don't even think about it.
For example, I could wonder why all the legal and medical shows do not have more Jewish characters, since a high percentage of lawyers and doctors are of that ethnicity. But, I generally just shrug it off. The only show that consistently portrays Jewish characters is Numb3rs. Yes, I know Saul is supposed to be Jewish, but that is so far from reasonable in that family that it boggles the mind. And Marion Ross as the mother of Nora and Saul? Yes, she is just about old enough to be Nora's mother, but nowhere near old enough to be Saul's. It was ludicrous. Unless Saul is the product of an earlier marriage of their father, which I guess is possible. I don't recall hearing that, but I don't often watch the show and might have missed it.
But, I digress. My points is that ethnic or other diversity in a show doesn't play a big part in whether or not I think it's good entertainment. To others, I'm sure it does.