I'm not sure if this has been addressed yet, but it occurs to me that when not in the leader slot that GL Luke will pretty much never get to use his Ultimate ability.
According to the text of the Ultimate abiltiy, Luke gets 8% Ultimate Charge whenever an ally gets Jedi Lessons.
As far as I can tell, Jedi Lessons is only applied in one of 2 ways.
1. Using Luke's Ability 2 (they grow Beyond) which grants a maximum of 1 application of Jedi lessons every 4 turns.
2. Using the Inherited Teachings ability which the team only gets when Luke is in the leader slot.
This means that it would take approximately 50 turns for GLuke to get to his Ultimate if he isn't in the leader slot.