Some arguments require you to make a long and detailed response. When you're discussing US gun culture, or rape in the media, or feminism, or the death penalty (just examples) if you're not making a well thought out response then everyone ends up parrotting slogans or only making one line comments that don't actually progress the argument.
Also, on occasion there is a very detailed fan debate going on, for which history and backstory is needed. I've seen comic book debates on these forums where each post resembles a Wikipedia page, but that's needed, and if you're a fan you'll probably enjoy reading it, and if you're not a fan, what are you doing in the thread? This is also the general attitude I take, if you enter a debate on rape and don't expect for there to be long arguments, then I'd argue that you have very unrealistic expectations.
This debate also has to include the question of 'what is too long?' I know some people who would have already stopped reading my post by this point, and I don't think I've been rambling too much. Sure if I wrote a several thousand word essay in response you have a right to say that I'm tling, but an argument presented in separate paragraphs, each paragraph with a clear theme linking the argument together? That should be read, I believe.
And of course finally the content and format should be considered. If in my above examples someone has written an eight page essay of spelling mistakes, bad grammar and unreadable nonsense, with no paragraph breaks and dozens of fragment sentences, then it becomes a tl;dr. Even a relatively short post can become tl;dr if it is badly presented, and a rambling vaguely racist manifesto should never be read of course.
So to sum up (tl;dr) There are situations in which a long and detailed post is necessary and in fact preferred to a short post, which will not advance the argument. To dismiss someone's post automatically because you can't be bothered to read it is rude and unhelpful. However, when posting, please make sure that the majority of what you say is relevant to the argument, and provide a summary where appropriate.