First off Tobi transplants different eyes. Example: His fight against Konan - He had to use Izanagi and lost one of his eyes:
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Another thing to remember is even though Kamui does (or "seems") to connect Kakashi and Tobi, Kakashi has a limit to his chakra without the Sharingan and when you add his Sharingan/MS with his most powerful move you get someone who would die of loss of chakra before he would go blind in that eye, if that makes sense.
To better explain this, look at the real Uchihas of the Narutoverse. They start to go blind after they use their techniques too often, etc.. Not only that but MORE IMPORTANTLY it is in their duty to get EMS. We all know you get EMS through taking another Uchiha (or the Uchiha's Best Friend/Brother/Closest Person) to unlock EMS. In other words it is kind of like a "right of passage" for the strongest of the Uchiha.
I don't think it is more so that Kakashi's Sharingan eye was transplanted, rather that he doesn't have Uchiha blood coursing through his veins, and since we don't know exactly how he even unlocked MS but we do know why he covers that eye up all the time unless NECESSARY, one can deduce that just like Kakashi has to cover the Sharingan when not in use because it drains his chakra, the same can be said when he is using it...He can use Kamui as many times as his Chakra can handle but I don't think he will ever lose vision in the eye because it is not a "right of passage" for him.
Also, Itachi, Sasuke, Madara, and all the other Uchiha's we have been shown started losing their vision after unlocking the MS and Amaterasu to Tsukuyomi and finally to Susanoo. Kakashi can't do this because one eye unlocks Amaterasu and the other Tsukuyomi.
I believe in the latest chapter Kakashi had blood coming out of his eye because he was simply over-doing it/running out of chakra. To put it in layman's terms: Kakashi's MS/Sharingan has to be covered up because he CANNOT deactivate it. If he were going to start going blind it would have happened already since it is never deactivated and always using a little bit of his chakra.
