I think for the most part, by the time someone posts on an internet forum for advice on their relationship, they are at a point of really needing some help. Often the OP isn't full of details about what is going on, but there will be tiny clues, or the right questions will be asked and it all starts to come out.
Most people don't ask for help after their first fight - they might post about A fight/argument/dispute, but this will be preceded by months, sometimes years and years of problems.
From my own experience, I am a professional personal, Post Grad educated, fairly worldly. Yet I couldn't see/didn't recognise the abusive relationship I was in - if you are in the middle of that kind of madness, sometimes you really just can't see it. I knew things were wrong, but I thought it was something I could fix or change if I worked hard enough, and XP just had a tough time/was tired/stressed/poorly parented. But with XP, like with many abusive men, nothing I do would fix it, and he was an abusive arsehole in his relationship with me. I don't know why and I no longer try to understand why, but it was.
I was fairly poorly parented myself, had no positive relationship models growing up and lacked emotional intelligence and self esteem. So while I appeared to be confident, intelligent etc, I was a perfect target for XP and my self esteem was so shot, and my self worth so all over the place, I didn't have a chance to identify what was going on. In hindsight there were red flags all over the place, but I ignored them. I didn't know what they were.
Time after time on MN you see the layers being peeled away, and then finally the penny drop and you see actually what is going on. I think unless you have been in that situation, you probably can't even believe it is possible. How can someone not see it? Well people don't - for whatever reason they don't.
I learnt a heck of a lot about myself and my own very flawed relationship via MN Relationship. And now I see people posting and I can recognise similarities in the situation and confusion straight away. You will see the knowledgeable posters asking the right questions, probing and getting the OP to slowly see what's up.
Alternatives to MN weren't really available to me. My family are emotionally frigid. I would have liked to explore issues with a counsellor but XP would never have stepped in to look after the DC so I could attend - or more like he would say he would collect the DC that day and then at the last minute say he couldn't/something had some up etc etc etc and undermine the whole thing.
And yes (cliche warning you might want to look away now) it did get worse after I had a baby and particularly after the birth of DC2.
It does make me cross when people who lack experience (but think they know it all and think they know better) wade in and start slagging everyone who is giving advice off when really they don't have a fucking clue as they haven't been in the situation themselves. Seen a lot of that on MN lately.
If you don't think it is useful, you don't have to hang out here - there are lots more boards and strings to the MN bow. But I think it is useful to lots of people in a very genuine way.
The Relationships board is not the place for fight club and some posters recently should be very ashamed of the chaos they have caused on genuine threads, started by real people, looking for help and insight.
Oh and if I had a tenner for every moody H/affair thread where the OP swears black and blue there is no possibility of an OW being involved, and then time passes and the thread rolls on and eventually it all comes out and there is an OW. It's astonishing - I've seen the very same thing happen with a RL friend recently too.
I don't see these phrases you quote being trotted out regardless of the OP. Perhaps you could link to examples?