Is Andrew Bynum considered an elite defensive post presence now? Even if he was, he played 17 mpg and 24 mpg on those two championship squads.
Having good bigs who can control the paint is important. Of course, having good guards who can attack the paint is also important. Something like 20 out of the last 25 champions have had an all-star guard on their team (or in the case of the 2004 Pistons, a deserving all-star who was also Finals MVP). That's more than the number who had an all-star big man during that time period.
(If we're going to treat all "bigs" as equivalent, then it probably makes sense to discuss guards the same way. Jordan, Kobe, etc., are all similar to PGs in that the ball is almost always in their hands.)