That's quite seriously not a benefit in any way which should influence the decision.
The stick-and-rudder aspect of flying the airplane is only about 10% of the mission...plus, unless those people are actually actively flying a T-6, they would require a requalification before being able to fly the hypothetical AT-6B.
In the fighter world, this non-tactical portion of the flying training is about 5 or 6 rides, even for a B-Course, before the Form 8 instrument check. Again, not exactly what you'd call any kind of significant benefit.
More significant would be the numbers of CAS and COIN experienced pilots who could enter the training pipeline, of which the other 90% of the training challenge consists of, and which it does not matter if the airplane is a Beechcraft or a Tucano.
The only somewhat benefit to buying the Beechcraft would be the ability to partially follow the established logistics system for the T-6, as well as economies of scale for the logistics system for the entire T-6 family of aircraft.