That's an incorrect assumption. This manga is the first one that Tobi has been seen using an MS. All of the rest of the time tobi had an original Sharingan. It isn't a plot hole.I hate plot holes so I have to point them out... Itachi's ms was rarely ever used, or so we can assume from the anime because it was dangerous for him. By the time he fought sasuke his eyesight was so bad he could barely see anything.
Take Obito's ms, tobi has been using it AT LEAST since naruto was born, more than likely since obito was killed. Now let's also assume that whenever he's been attacked and lets them pass through him, whenever he's used the other dimension (which is almost constantly in the manga) he needs to activate his ms. How the hell has that eye not gone completely blind?
I'm sure the answer is plot no jutsu but that crap annoys the hell out of me
In Tobi's fight with Minato, Tobi used s/t jutsu several times and the manga dipicted Tobi with an original Sharingan. Kumi needs the MS but s/t does not. Just like Itachi can use genjutsu with an original Sharingan but has to use the MS to use Tsukuyomi.