This is a response to the notion that Tsukoyomi is a unique ability only to Itachi, and that MS users are only capable of learning certain techniques exclusively for themselves.
In order to do this I will define what Genjutsu is. Genjutsu are illusory techniques that use various means to reach and disrupt the flow of chakra, and manipulate, bind, or trick an another person, usually an enemy. Genjutsu broadly falls under Yin release techs because of its power of a person mind, or spirit/soul.
Next is to define and explain the Sharingan. The Sharingan is a potent eye unique to the Uchiha clan, although others can implant and use one as well, that is inherent with powers that allow a user to learn techniques through simply seeing them once, cast powerful genjutsu, and manifest special techniques, such as Amaterasu.
Finally, I will define and explain Tskuyomi, and give a defense for my reasoning. Tsukoyomi is potent genjutsu that can only be used with the powerful eyes of the Mangekyo Sharingan that allows the user to manipulate space/time inside the illusion in order to add longevity.
Now my thesis is that Tsukoyomi is not exclusive to any one, or few, MS users.
This is my reasoning:
1) Tsukoyomi is a MS genjutsu that has been seen first, in terms of Naruto history, from Madara Uchiha since he presented the infinite Tsukoyomi to Obito as his program for world peace.
2) The fact that Itachi can control space/time within the genjutsu does not mean that the technique is exclusive to him. Sasuke learned to manipulate amaterasu flames and gave this this technique a name, but that is all it is because any MS user, with practice, could potentially control the same flames as well. Giant rasengan is still rasengan, just bigger, but it does have a different name thats it.
3) Therefore, with practice and a leaning towards genjutsu, the MS user can most definitely learn Tsukoyomi, and this makes the Tsukoyomi an ability that can be learned since potent genjutsu is inherent with the sharingan.
The notion that each sharingan gets a unique technique is also invalid seeing as how one sharingan is used to cast one thing while the other does another thing. This of course proves nothing, so lets look at two cases; Sasuke and Obito.
First, Sasuke has been seen using each eye uniquely to execute a technique, namely Kagutsuchi and Magatama. Kagutsuchi is simply casting the Amaterasu and manipulating their shape through the other eye, since sharingan techs are only executed through the sharingan, not through hand seals. However, some will claim that this is a unique technique because of its relation to a shinto god, but I beg to differ since shape manipulation does not constitute an exclusive tech that only one person can use. In fact, Naruto added his nature element to his rasengan to and it was called a different name (Rasenshuriken), but this does not mean that no one else can learn Rasenshuriken. Same goes for Magatama, although this technique seems to be used by Susano'O instead of being cast from Sasuke's eyes.
Secondly, Obito and his Kamui. I've never heard anyone bring this up, but since it seems that Kakashi's and Obito's Kamui differ in some respects, one could come to a conclusion that this is an example of two different techniques from two separate eyes. However, this is easily refuted on Naruto Chapter 608 where Kakashi is seen using his own Kamui to teleport back to where he was. Also, the fact that Obito's eyes have been shown to display Kamui doesn't prove that it is exclusive to him. For this I will appeal to an argument from silence until an adequate explanation can be made for the Kamui.