If someone expresses an opinion in a manner that is aggressive, rude, or offensive, I agree that there is nothing constructive about it and they should keep it to themselves or be ignored.
Hmm. I appreciate that this may be a goal to strive for, especially online, but as stated you know this is totally wrong, don't you?
Does the value of the message depend on the manner in which it was stated? Do you only accept feedback phrased in a way exquisitely tuned to spare your feelings? Do you only accept input from perfect human beings, who know how to present their feedback in the most careful way possible?
Can you see how much that limits you?
I get a lot of feedback in my professional life, and I've developed a fairly clear methodology for responding to it:
1. Total fury about what an unmitigated jerk the guy is
2. Piss and moan about how they didn't understand what I was doing or and how they didn't even try to read it
3. Complain about their ignorance and incomprehension about the subject matter
4. Object to the stupid way they phrased their comments
5. Sigh, put their review in front of me, and start to deal with it to improve my work
Trying to skip steps 1-4 never really works. Knowing that those are just stages and I've got to get to 5 sooner or later does.
Same if you're feeling put on the spot by Aikido discussions. So what if someone has gone into sales mode? That's their problem. Take what's valuable and leave the rest. So what if they don't think your Aikido is valid? They're a voice on the internet. Take what's valuable and leave the rest.
Or... if the comments are coming from someone you're not willing to ignore... take a long, hard look in the mirror and decide what you really care about. Decide if your self-image as a kick-ass martial artist is really worth preserving over the reality of being a kick-ass martial artist. You can't have the second without being willing to put the first at risk.