Nothing about minato haveing sage mode is a asspull or plot hole. NOTHING.
Perhaps, he is only able to use senjutsu for only 1 technique, then he runs out(absorbing to little senjutsu chakra....the opposite of jiraiya)
But I pretty much agree that it was pointless for kishi to have him use that technique now, and just get fodderized.
(logically it makes sense. Before naruto and sasuke were doing most of the fighting, and had the senjutsu chakra. So he didnt need to use his. And madara being very experianced vs ftg, and being a stronger juubi jin, is just that much more powerful.)
But it wasnt really pointless either. Just showing us minato learned sage mode. It was pretty pointless, in its effect, because he didnt really do anything.