I agree - there are several big advantages to the shorter head lines and fishing heavier tips is definitely one. Another that wasn't mentioned was standing under overhanging limbs tight to the bank (anybody fish the Sol Duc?) and trying to fish a seam 60-70' out. You don't need much of a D-loop with those shorter head lines - just flex the rod and the line will go out. With the longer belly lines, you need to throw that D-loop behind you and that can cause problems in tight quarters. I still keep my WindCutter's spooled for those moments!
But I think I've found my own niche with the longer midspey lines (70-80' heads) bacause, as the other Brian said, when you really need to reach out you can do so with minimal stripping and you still have a manageable length of belly when fishing tips. I've also found that, when I need to get down, 1.5" brass tube flies on my regular 15' type 6 tips will put me right near the bottom in all but the heaviest flows. The big advantage is that they don't affect the cast as much as you'd think, so I can continue to fish my normal lines even when the need arises to DReDgE (gotta do that a lot for the winter hatchery rats).
And the funny thing is that, with lighter wire hooks, I can be bouncing off rocks all day and not hang up near as much as with standard weighed patterns.