When Shimano unveiled it's 2x10 drivetrain, holdouts to the old 9x3 were a little tough to sway, but then they started to recognize the efficiency and weight savings inherent in the system and started to move over.
Since then, the industry has fully adopted the beauty behind it and it's tough to beat the full spread of gearing that you get from just a two chainrings and a larger cassette spread.
The XT Shadow rear derailleur is meant to be the meat that drives this phenomenal setup- the Shadow part is just the icing on the cake that keeps it all in place. The 10 speed cassette spans such a wide range that it's tough to get any better and this mechanize drives it all with quick and precise shifting.
Efficiency was the key principle when designing the Shimano Dyna-Sys XT M781 rear derailleur. Designed with a longer swing arm and a tighter spring for crisp shifting when you want it, and less 'ghost shifting' when you don't.
The 781 will stay in adjustment longer and deliver optimum performance with each ride.