why don't you just collect the set of joints specified in the joint trajectory method?
I would prefer to get further away from the requirement to specify joint model groups everywhere in MoveIt and this somewhat counteracts the effort...
Also, you still have to consider overlapping JMGs during checking, so I don't see the advantage of having the string here. 😕

That might be better discussed in the original PR, but in a nutshell, we did it this way because when we check for collisions between the two trajectories here, we need to give the group_name to the PlanningScene's isPathValid in order to limit the collision checking to that group. That API continues via isStateColliding and checkCollision all the way to the CollisionRequest in collision_common.h, which does not accept a set of joints. Opening that up is definitely out of scope for this PR.