By setting up Myshkin as a Christ figure, but not giving him supernatural or divine powers, Dostoevsky tests the possibility of a "perfectly beautiful" man existing in the artificial and duplicitous present. As expected, the experiment sheds a bright light on the way society has moved away from the virtues of humility, simplicity, and love for fellow man that are the tenets of Jesus's teachings. Nowadays (in Dostoevsky's time, that is) it only serves to make Myshkin a laughingstock. The Idiot's ultra-dark ending suggests that only the very young, who still have the capacity for idealism, can be saved.
Myshkin's selflessness, as shown in his love for Nastasya, is actually just as harmful as the selfishness of those around him. Even if he had succeeded in saving her, he would have hurt a much larger number of people.
Myshkin is less useful as an ideal for the other characters to aspire to, and more useful as a way for the novel to demonstrate the intrigues of a small circle of people through the eyes of someone who refuses to cooperate.