Ritsu was great when she was first introduced, but as I go on with the series, she seems to be a little less useless every time I see her, and then in the past few chapters, she was just gone. Is she truly a great help? When they went to that one place to make a major Assassination attempt, I noticed that Ritsu didn't take part at all. So what I'm asking: is she a necessary component of the class?
Bear in mind that there are a lot of students in E-class, and as such the author has the tendency to rotate characters in and out of focus. Ritsu has her moments in, for example, the Assassination Island Arc and the God of Death Arc (not gonna discuss in depth to keep the answer spoiler free.) The only students that have more of a consistent focus are Nagisa and Karma.
To answer your question, her necessity as a component of E-class is more or less equal to any other supporting character, for example Ryouma Terasaka.