I know that ITV's Midsomer Murders got slated a few years back, for not having many ethnic characters. The producer at the time, Brian True-May, suggested that was due to the fact small country villages did not really have metropolitan communities, which I believe he is right.
Needless to say, that same programme, under a different producer, now has a significant number of such characters, and lost any realism it may have had.
Programmes set in more metropolitan areas - generally the big cities - should have a proportional spread of characters.
As said earlier, having positive discrimination will alienate many, and undo all the good progress that has been made over the last couple of decades, be it colour, gender, sexual orientation, whatever.