I think the content of the comic is important for context of when it was made (on that front, the animated show removing it will naturally create a very different tone). However.....
It's very difficult to get your friends to want to read it if you all don't say slurs and have to Skip over Karkat's ten billionth R-Slur while you all do voices. The Slur Replacement mod is really useful for that purpose- I don't want to be tripped up by a sentence and have to stop in the middle to come up with my own word, if I'm reading it out loud in a character voice, when the mod can do that for me! It may not be the intended way to read, but I think it serves it's purpose.
People bemoaning that it exists AT ALL sound a little bit like anti-SJW types tbh. Nobody has to install it if they don't want to, it's just there for people who do!