I don't recall saying anything like "you don't need a reason to include a white able bodied person"- I hope all characters have some kind of purpose! If I were writing a story, say, about the Blackfeet in the 1860s, white people of any flavor, other than traders and soldiers, would be odd and one would definitely need a reason to toss one in.
Of course, as I think I mentioned a time or two above, people can write whatever they want. The original topic was our pet peeves and mental illness, so I guess we are all free to share those peeves, right? I've stated mine. You are free to disagree. In general, I think mental illness is portrayed poorly, both providers and clients, with some exceptions that i think are done well. Again, it's a matter of opinion and that's mine. People don't research, or use dr google or hollywood or something. They create cardboard characters with some fancy diagnosis that has nothing to do with the plot. I don't see that as any giant favor to anyone struggling with mental health issues. And that I suppose owuld be a whole separeate topic--whether there is some kind of obligation on the part of the author (I don't think of it as an obligation but I dont' like it when it's not done authentically--people do take stuff they read in novels as "truth" about things, including mental illness)
And yes, in my opinion, it is just like lawyers because while there is definitely variation and plenty of it, when someone gets it wrong--the disorder, or the experience, or the treatment, or the provider, it's incredibly distracting. Not getting symptoms right, not getting assessment and testing details right, not understanding the difference between psychologists and psychiatrists, having providers develop social relationships with their clients, describing completely inappropriate treatment, etc. If it's done well, I won't notice.
Again, examples of where, in my opinion, it was done well are both Prince of Tides (again, plenty of issues with the tx angle but definitely, Savannah's mental illness is a critical plot driver) and Beach Music (one person's mental illness results in an action that is a major driver of the plot.) It isn't having someone with a mental illness, it's giving them specific symptoms regardless of the diagnosis that result in actions that drive the plot. Sue Miller does this in Family Pictures where the MC's bro is...autistic? I can't remember the specifics but how the family relates to him, and what happens to him are crucial plot elements. John Lescroart gives his MC a daughter with some kind of anxiety disorder. Her travails sometimes distract him, sometimes impact his relationship with his wife and so on, issues which definitely influence the plot. Same with Ordinary People--the MC's symptoms are integral to the plot and are really well done.
Anyway, really, those are my opinions, and just that, opinions. I don't know why one would create a borderline character with no plot relevance (why borderline? why not conversion disorder? or avoidant personality disorder? i would guess because something about the borderline PD is plot relevant), but it doesn't matter what I think about it. No one needs my approval!!
Again i'm quite okay that we disagree. These are just opinions!