Goran Anderson uses 25' leaders. The long leader acts as his anchor. Having watched him a few times you will note that his line doesn't touch down on the anchor - only his leader.
I think this is longer leader idea is critical with short head dry lines like the Loops. With a 12-15' leader the possibility of pulling the anchor while forming the d-loop would be great, the 25' leader mitigates this problem nicely. With lines that are a little longer the extreme length is probably not as necessary, but the theory remains valid.
With my extreme long belly lines I usually go 2 or 3 feet longer than the rod I am using - as I don't usually have issues with pulling the anchor!
Tight lines - tyler.
Still Living Large!