TL;DR is for crappy writers and/or long-winded comments, which is why you see it a lot. It's merciful to the reader to put it up top.
Screw the conventional construction of an article. TL;DR isn't a spoiler either, because most people don't write comments in a coherent linear way like a good story. It takes most people three paragraphs just to get to the meat of what they're trying to say.
If you feel you need to use a TL;DR, you are a crappy writer and/or have written an unnecessarily long-winded comment. Put the TL;DR up top. Either that, or rewrite your comment.