Two additional thoughts.... One is technical in nature and the other is more social.
TECHNICAL - I came of age in terms of "forum" when they were called "groups" and you read groups hosted on usenet servers using Unix tools like tin and trn with either vi or emacs editors. Quoting attributions was a HUGE deal back in the day and have actually worked on a scholarly article on the use of quoted attributions when using mailing lists for collaborative work. The OP is spot on in his call.
There is, or was, a parallel debate surrounding email clients usually posed as "top-posting vs bottom-posting". Old unix email tools like elm would "bottom-post" by default, placing the cursor UNDER the quoted material and making it easier to trim attributions. The advent of windowing operating systems destroyed the assumption of 80 character line length and made word wrap the default and with that, trimming attributions in email clients became much harder. Microsoft Outlook and other mail clients moved to top-posting by default with quoted material being carried along in big blocks or mail chains at the bottom of mail messages. For many years, you could find freeware add-ons like "Quote Fix" to trick Outlook into bottom posting.
A similar thing is currently happening with the rise of the iPhone and tablets. Most PHP based web forums don't fully support tap functionality (you can't resize the editing window) and real estate is limited on the display. We saw the same problem on a mail list I used to moderate. When the iPhone hit, it became a LOT harder for people to trim attributions.
Sorry for the technical walk down memory lane but the short version of this is that changes on the client end are driving differences in user behavior.
My personal response to this at this point is that bandwidth is cheap. When I post from a laptop, I trim attributions because it's relatively easy and I think it's the best way to respond. But when posting from tablet/phone, I often just bottom post without trimming anything. It's just too hard to do.
SOCIAL ISSUE - Forums allow us to make friends. I have friendships that date back to the 90s from usenet. There is danger though when friends "talk" on public forums. Outsiders perceive it to be a clique. This is the forum equivalent to walking up to the school cafeteria table when you're the new kid, or walking into a new bar when you're older.
As Jack correctly notes, the thread in question is a part of the core of the sub-forum, just as the "regulars" are the core of a good bar.
I'm not sure where the balance is between good bonding among friends and discussions that end up being a cliquish form of exclusion new comers. I rather doubt that the real issue here is untrimmed attributions but rather, the recurring problem of "chattiness" and "me too" posts.
My sense is that the right mix between familiar chumminess and inclusion of the new comer is like the fuel/air mix on a carburetor. I know it's never intended this way, but I think OP's post is a good reminder than when we are responding to our friends in a familiar way, it can be off putting to others. That's the danger of the "me too" or "Nice shot" post.