Hey Ryan,

The "why" is:

Better resolution
More torque to the control
Perfectly linear control throw
Less hinge and servo wear
Less cost
Increase in reliability
Ease of maintenance

The real question becomes "why not?" Honestly, for those who have tried the system, the only reason they ever go back to linkages is for CG issues. That is, some kits are tail heavy to begin with, so placing servos in the tail may compound the problem.

Stek79, if you are still around - your argument is absolutely right. It does, however, ignore the issue of deflections due to loads. We are not dealing with rigid materials, but rather flimsy balsa, plastics, and rubber grommets. Your linkages will have more resolution than direct drive...on the work bench. Add some load to the control surface, though, and direct drive will beat it every time. Here are some of the "extra" loads a linkage creates:

The push-pull loads created by a linkage act on the servo arm, hinge, and horn.
A ball link adds a twisting force to the servo arm.
The angles of a linkage at full throw cause odd loads to the control, and reduce the linkage efficiency most when the most torque is needed, i.e. when it is deflected the farthest.

A linkage can only add play to a setup - play that is not there with direct drive. Essentially, direct drive allows you 3D throws with better than pattern resolution.

John