That could theoretically work, but thats putting radagast at pretty serious risk to accomplish something that could be done just as well with a throw-away unit.
Even if I win the roll for epic action priority in the charge phase, he is still vulnerable to black dart at that distance.
Really, just being able to move 4-6 companies of riders right in front of their line on turn 1 buys me alot of time. They will be moving at half speed for me being close, and if they choose to charge them, then its really only better for me, cause they are effectively not moving at all. Plus, if my opponent doesnt think, I might even be able to set up some flank or even rear charges if he tries to envelop the unit.
Or, He ignores the unit, and then I have a small pile of riders running around his deployment zone being jerks.