I think former players with a bit of a brain and a good mind for the game usually make decent commentators, particularly captains, as shown by the likes of Benaud, Chappell to a lesser extent, and more recently Hussein, who is turning out quite well. Taylor shows that this doesn't always apply, though. However, cricket players aren't always the most naturally attuned to the requirements of broadcasting, and aren't always the most eloquent or interesting guys either, so it doesn't make sense to assume that only ex-players will be fit for the positions. As you say, there's plenty of good examples to suggest otherwise, while the likes of Healy, Jones and Botham played the game and yet are pretty terrible at calling it.
I think the ex-players and colour commentators policy in Australia is a Packer-related thing. The guy he put in charge of revamping the 9 coverage (forget the name offhand) looked for a simple, "family-oriented" presentation of the game, which basically translated into recognisable ex-players who don't make it seem too complicated. Hence, Bill Lawry.