There's a difference between discussion and, well, ranting. It's one thing when you dislike a feature or change, and point out that "Yeah, I don't think this was a useful idea", and then proceed to explain the pros and cons, as well as alternatives.
When the discussion devolves into insulting and attacking, becoming so filled with hate and anger such that constructive or productive material is easily being clouded by ranting material, it's not useful to have on the site. That thread was mostly whining. I don't deny that it was a problematic change - it came out spontaneously and removed a feature that many people used frequently. Posting image macros with the intent of insult does not help. Posting over 12000 characters of just complaining that "This was bad, you're mean, you lie, you are dishonest, this place isn't run by the Community" is equally going to be ineffective. To quote a reason for closing and locking a similarly inspired thread:
"have you stopped beating your wife yet?" rants will be closed and locked. Discussions are welcome. – Jeff Atwood♦ Mar 11 at 12:10
Jeff Atwood's answer to that question was a combination of "We are in the middle of improving things, please be patient and give us feedback on the system as we roll it out as well as any features you would like" and "The envelope was detested by many, so we're rebuilding it to something better. You can still access it." Rather than actually take that offer and provide feedback, the vast majority of the thread was more concerned on bashing Jeff and pointing out all kinds of past statements. There were some people who actually provided feedback, but not nearly as many as those who were just raging.
Shouting at people doesn't really make it any easier to get things done. If you're hurt, explain it. Tell what features you missed, and what features you think should be reimplemented. Then it will get looked at and probably implemented. Some people did that, but it got lost because of all the people who were fuming so badly that they didn't let up for even a second.
To answer your question? You can discuss topics, even features that you don't agree with. It's not even like the Team never changes their mind - we've had status-declined reverted and a multitude of things that get implemented which were previously strongly opposed. But if you're not going to discuss, and just going to incite basic wars... it's not useful to anyone here.
It's not useful to the Team because it's just shouting, it's not useful to the ranters because it won't get their needs addressed, and it's not useful to the neutral parties who actually want to try and improve the site.