Huh? What's there to cover with your point? SM is not needed in his fight against Obito. We all know that they are battling with speed and there is no faster than FTG which is instant. With regards to Juubito, it's your point of view that it was needed because only SM can hurt Obito but you are neglecting the fact that there is already an SM user which is Naruto.
We can't gauge what's more powerful between a BSM+SM or BSM+BM but we do know that BM is more powerful than SM. Well at this point, I can't say you are wrong because it's your opinion but I believe that a BSM+BM combo is more powerful than BSM +SM. Let's say Minato uses SM. What will he do then? Obviously he will be adding his senjutsu chakra to naruto's TBB which I believe it's not needed because Naruto can add his own senjutsu chakra. How can Minato add some power to that? If he is using BM then he can give more power to their attack than by adding only senjutsu chakra in SM. On top of that, he is helping the alliance with chakra. Talking about usefulness, I believe BM is more useful in that situation.
I think your underestimating the usefulness of two senjutsu users against Juubi Jin Obito. Given Minato's incredible speed with FTG, he could have used senjutsu Rasengans in conjunction to whatever attack Naruto and Tobirama would use against Obito. It was evident enough that BSM Naruto still was not enough to defeat Obito seeing that Sasuke stepped in as well.
Opinion or not, if Minato had entered Sennin mode at the appropriate time, there's a high chance that Obito would have been taken down much sooner, which can only mean that he did not have a sage mode a couple chapters ago. Also he wouldn't have to add senjutsu chakra to Naruto's TBB because Naruto could have done that on his own.
It's the effectiveness of said power-ups, out-of-the-blue or not. And besides both Bijuu modes were tossed aside when Obito released that gigantic tree. If anything their giant Bijuu cloak sizes made them easy targets to take being that they were both drained when branches wrapped around them while Killer Bee was free to move around slashing tree branches. Had Minato not been so large and in Sennin mode that could've been avoided.
And plus, Minato may have been feeding chakra to the alliance like Naruto, but they were both still very much capable of fighting while doing so.