There are a couple reasons for this. Generally speaking, a turned left hand equates to a slightly more forward leaning shaft at impact, which delofts the club and adds distance.
Also, there are power advantages in allowing the cupped left wrist at the top, which is in contrast to what Trevino and Pressel do, then keep the wrist flat (or in Trevino's case, slightly arched) at the top - this is the root of why their power is not maximized using the 10-2-D grip (and why their accuracy is).
Couples and Daly use CF to square up the club. They both have excellent Rhythm, which combined with the cupped left wrist at the top (and the ability to retain their angles deep into the downswing because of the cup) contribute to their distances.
So if you use 10-2-D as an angled hinge swinger, you can still have power, given good Rhythm, but 10-2-D as a hitter is generally less powerful (because as a hitter, they do not allow the cup at the top). Depending on the player, the more 'massive' impact of a 10-2-D hitting motion can make up for 'most' of the distance differences, but never all of it IMO.
Good post. I think that handicap players could possibly benefit from 10-2-D if the required release motion (hammering) is understood. It eliminates some of the moving parts and helps negate throwaway. Most amateurs suffer loss of distance from throwaway and leakage. So 10-2-D could be a "stepping stone" grip.
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