Commenting with emphasis in all caps is the textual equivalent of grabbing someone by the collar and screaming in their face.

Everyone who does so feels it's justified, but the effect is always obnoxious and it never conveys more information or meaning, only an insistence that the commenter is especially important and needs to be paid attention to.

Very common way people try and "win" on the Internet is by arguing a point not made by the parent. It wasn't all in upper case; your comment is invalid.

It's been considered rude since the days of Usenet and probably before, and whether or not the comment was entirely in all caps is irrelevant.