That's a hell of a stretch. Also, technically speaking Ninjask is not scarfed, and I specified Scarfers. If put on an even-item level, Diggersby is a bit stronger on the U-Turn.
Choice Scarf Diggersby DOES provide a "Best of both worlds" scenario, however. He does scouting very well, and he does Revenge Killing very well. (Also, CB Diggesby has an iffy role as a revenge killer. He loses speed, which makes him entirely reliant on Quick Attack, so reasonably fast Steel, Rock, and Ghost Types will walk all over him.)
and yet he outspeeds adamant scarfed diggersby, which is huge. Here's Jolly scarfed diggersby.
252 Atk Huge Power Diggersby U-turn vs. 252 HP / 0- Def Abomasnow: 254-300 (66.1 - 78.1%) -- guaranteed 2HKO
That's about a 10% drop in damage
Jolly scarfed diggersby happens to outspeed adamant banded ninjask by a mere 1 point, so while that is significant, I would not be calling that a "hell of a stretch" as they have very very similar speed tiers.
Obviously jolly banded ninjask outspeeds jolly scarfed diggersby, but unless you really want to outspeed some scarfers, there isn't much point (I'll take almost ohkoing the eon twins, thanks).
What a lot of poeple don't seem to realize is that overkill speed allows you to have the speed of a scarfer without an item, which is significant.
As for "best of both worlds" argument, I can't imagine to many relevant fast steel types, he outspeeds heatran which is enough for most of them. Scarf does allow him to revenge terrakion, which is nice, but besides that there aren't too many rock types diggs particularly fears either, outsped or otherwise. Scarfed or unscarfed, he does not care for ghosts, best case scenario is that you are shoehorned into wild charge, which is pretty predictable and lets the opponent get a free switch to his ground type or volt absorber of choice.
And yeah, as Alex said, exca and lando-T probably do that better anyway.