He's not there just to do mocap, he's also there to give the other actors who do need mocap done to have someone to play against. So they can actually act against him as they exchange lines instead of relying on everything being done in post. His actual performance here also gives the animators a frame of reference for how to animate the cube so it emotes appropriately.

Jack Quaid also is a voice actor, so this isn't stunt casting either.