Shisui's Mangekyou Sharingan casts the mind-control genjutsu, Kotoamatsukami, but it activates only once in a decade. Why are people so eager to grab his eyes?
Several big spoilers ahead.
Three people have been showed to be eager to grab Shisui's eye, namely Danzo, Tobi and Kabuto. Besides that, they have one other thing in common:
They all have access to Senju Hashirama's cells, which significantly reduces the reactivation time of Shisui's eye. Hence, these people could put it to practical use without the undesirable restriction.
Danzo's case actually shows this reduction of the reactivation time. He had stolen Shisui's right eye shortly before the Uchiha incident, and had Orochimaru implant Hashirama's cells into his body. He was able to activate Shisui's eye twice in a day, first during the five Kage meeting and later towards the end of his battle with Sasuke.
Tobi is later revealed to be Uchiha Obito. The right half of his body, destroyed during the Kannabi bridge battle, was repaired/replaced by Madara with Hashirama's cells.
Kabuto had access to Hashirama's cells due to both working under Orochimaru previously and through Yamato whom he had captured during the fourth shinobi world war. Shisui's left eye emerges during the war from Naruto's mouth due to Itachi's setup. Kabuto also hears Itachi talk about the use of Hashirama's cells to reduce Kotoamatsukami's reactivation time. He hoped to grab it for his own use.