Forums are usually a place for information, opinions and discussions, threads base upon these things. You are here to read just as much as you are here to post. I can understand that not everyone enjoys to read long posts, especially depending on what subject it holds.
As was mentioned above, at times it can be a pain to read through most of these long posts that you can bump into, and even worse so if it's just not basic information but actually various opinions from different users regarding a certain subject. In some cases it can only be the matter of a user/person wanting to make as a much elaborate explanation or opinion, or in other cases maybe even that the subject at hand cannot be cropped down too much without sounding completely flat.
For a reader the text written by another when it becomes a bigger post must have some interesting writing(typing in this case) in order for people to keep on reading it. You as also want to make sure that others truly understand what it is you are trying to point out to the masses. Some matters are just more easily understood this way. Of course times might call for you to make short posts too once in a while, but in general, a long post as long as it's interesting and engaging is the way to go on the forum. Now as you said yourself though with users posting long posts and already saying everything you might thought of can happen to all of us, but when you think of it. If this "person" wrote a longer reply to a subject, and you fully agree with it and cannot come up with anything more to add, doesn't this mean that the poster actually did a good job with their post and got their point across?
As much as forums are for speaking for you and your opinion, it's also about speaking for others or balancing facts and information for others to read. I'm sure that if you gave these longer posts a good thinker, you'd be able to give feedback on those upon the original subject or maybe add on something new. Many threads on the forum end up big just because users discuss over other users replies and thoughts, not only for what you can see in the top post of the thread.
I also wish to point out that posting shorter posts after a longer one is not wrong if the thread was made to ask for opinions. There's ways you can explain your thoughts in an own way without actually copy pasting what another wrote. Could be seen as creative typing if you want to call it that. I will go ahead and excuse for this longer post due to the subject of this particular thread, but it's kinda to prove a point as well, not certain how this would look in a few sentences.
