Dealing with words is what we do here. We're not an image board, where people react to each other with images and cliches. We engage in writing and reading. Emoticons are supplied to help make intent clearer.
Another thing to consider: we prefer that threads can be revisited years later and still make overall sense. Reaction images aren't archived locally, so what happens if a link breaks for whatever reason? Posts cease to make sense, all because somebody couldn't just put their thoughts into words.
Also, I have no idea why a "long paragraph" would be necessary to express the sort of thing that people use reaction images for. Usually, the content of the image can be summarized as, "I find this humorous", "I can't believe something like this was posted", "I find your argument unconvincing", and so forth.
And no, i don't think it's really necessary to "tailor" some image for the specific discussion of the thread.
Of course it isn't "necessary". But the results can be quite a bit more entertaining than something regurgitated from 4chan or tumblr.
Overall, my take on the matter is very ambivalent. Part of me wants to say that it's okay to use them, but only in extreme moderation, and never to "say" something that you know you couldn't get away with writing. (A good number of reaction images serve no function other than to visually demean.)