Geoff is a great and Charismatic speaker but he is not going to want to take the stand. Vin Toole is also a great charismatic speaker, and look how dejected he appeared. Whilst Geoff will do a better job, he still has nothing to gain by getting on the stand.
Having Geoff testify will be like pulling back the curtain in Wizard of Oz. The sheep are in awe of the GB and their direct connection to God in making decisions. McClellan will tear that apart and show Watchtower is made up of committees with majority votes. Geoff's appearance won't help build anyone's faith, but it will remove the mystic for some Jehovah's Witnesses.