I think you ignore the fact that there is a process or a narrative (or whatever you want to call it) about coming to terms with being in a dysfunctional and unhappy relationship. I am a strong, independent person - but I didn't even realise I was unhappy in my previous relationship for years. I was young and it was all I knew.
When some people post in 'Relationships' you can tell that they are at a kind of transformative point in their realisation about their own position. They often cerebrally know that they are being taken advantage of, but their emotions haven't yet caught up. Sometimes you can actually hear this - a very large number of people will say something along the lines of 'This isn't OK is it? But I still love him. But it's not OK.' You can tell that, by the very act of writing about it, the poster's views are changing, but that they haven't come to that moment of making a decision.
That's not martyrishness - it's confusion, emotional struggle. The advice and support that is provided is often given (whether consciously or unconsciously) to help bring about enough of an alignment between head and heart that the poster can take meaningful action. I don't believe that anyone posts on Mumsnet relationships that their partner is emotionally and physically abusive, thinking that they are going to get people advising them to stay! So there is a kind of tentative 'trying out' of a new way of looking at things going on, and that's important.
I actually think that sometimes the board can go too far - that there can be relationships that are equivocal or where there isn't that much evidence, and there will be a chorus of 'LTB' which maybe doesn't do justice to all of the grey areas of human experience and all of the ways in which relationships can be complicated. On the other hand, there are many cases where a woman's situation is absolutely black-and-white, and she sees it as grey and minimises. It's a tricky balance, which is why it's good that there are a range of views expressed in those cases. In cases of serious and unequivocal abuse, though, it's very, very rare to see posters offering anything but support to leave.