Sean, I think you're confusing the factors contributing to a player's handicap with the factors contributing to a course's difficulty.
It's true that low handicappers are better than high handicappers for a lot of reasons, and that length is just one of them. However, when it comes to predicting the difficulty of a course relative to a player's ability, there's still no better predictor than length.
That doesn't even mean that length is the primary facet of a player's game being tested by those courses. A 375 yard hole is reachable in two for just about every male golfer. It doesn't "test length" per se. However, length is still a HUGE advantage. The long hitter gets a drive and pitch. Even poor pitchers hit the green around 50% of the time from inside 75 yards. The short hitter, on the other hand, gets a drive and 3w. While he has plenty of distance to reach the hole, he's still unlikely to actually hit his target from 175 yards with a 3w. The fact is that players are just less accurate from longer distances, and thus being able to hit the ball a long way, thereby shortening the distance of your next shot, is always valuable.
Lengthen a course, and everybody is facing longer shots with lower margins for error and therefore shooting higher scores.
The benefit of being a long hitter isn't just that you can be pin high more often. It's that you can be closer to the pin than another player almost all the time, and thus always facing easier shots.