It is cathartic in the sense that Holden is, in his interior monologues, a rather uninhibited exemplar of dissatisfaction and disenchantment with the system. All of us at one time or another have felt like outsiders and have been resentful of the “cool kids” who always seem to fit in (to quote the song from a few years ago by Echosmith). Holden is an extreme case because he dislikes most of the people he comes in contact with. The teachers and the other students all seem like phonies to him, and he expresses his opinions of them in uninhibited (for the the 1950s) language. Most of us can identify with his feelings to an extent, and it can provide a kind of release or catharsis to listen to Holden.
Holden says things to himself, and at times says them openly, that others might be thinking but are reluctant to express. His nose-thumbing at authority figures is simultaneously comical and insightful into the subversive part of most people’s inner nature. There is a difference between this and truly anti-social rebellion. If Holden were engaged in open rebellion against the establishment through criminal acts, that would obviously be much more than most readers would identify with. His behavior is a balance between non-conformism and pretending to conform. This is, perhaps ironically, what most of us who do at least occasionally question the powers-that-be of society can relate to, allowing us feel a kind of release of our own inner thoughts when listening to Holden’s monologuing.