Are you saying that Optimus isn't humble enough to lend his power to his friends for their benefit? If others have combined with Optimus before to power him up, shouldn't it be only common courtesy for Optimus to take a turn at powering up someone else? He's a team player.
Of course Optimus would gladly act as a belt and backpack for his comrades, he's just that kind of prime, by why would TakaraTomy even ask such a thing of him? And you literally can't tell he's even involved, he could have at least been visible from the front!