Eh.. If I need to pretend to be something I'm not just to be "accepted" then I'll rather not be. In most cases when people are changing their behavior to meet the expectations of others it's actually their own imagined expectations they're trying to meet rather than that of other people. As long as you're not causing harm to anyone, most simply don't care if you're a bit weird.
It is ALWAYS bad advice. "Be yourself" is the most pointless thing to say to anyone. It doesn't help the recipient at all. Same for "be a better person". You could say "be genuine" and that would be slightly more helpful.
From my background in education: under-performers tend to be poor judges of their position, tend to have no idea what good performance looks like or how to get there, and tend to surround themselves with similar under-performers. So someone who is underperforming in a social role (making friends, fitting into their work place, dating, etc) needs a lot more focused and good quality feedback.
Be the best version of yourself. We are all a product of our environment and opportunities. We are all animals. We are all evil under certain circumstances. All any of us can do is be the best version of ourselves.
The advice is/can be used in a ton of different situations, you seem to only think it's for a very narrow and small part of those. And even then, it's not always the best thing, someone can be themselves 99.99% of the time but sometimes it's better to hold it back. Even something as simple as if they're meeting your parents for the first time and you know there will be problems if you don't tell them to not mention something, act like they think differently about something etc.
This very much isn't just about a possible partner for you and all the time. I don't understand how you thought that was the only thing. It's like if someone said a common feature of cars might not be worth keeping on all the time and you read that as only a specific line of Volvo cars, in very specific situations and like you can't turn it on and off. See how weird and dumb that is?
Pal, the only one thinking about "partners" here is you. I was referinng to human interaction as a whole, friends, coworkers, family, etc. "Be yourself" is and advice to seek your place among other that likes how you are, whether you're a piece of shit or a saint.
Although, I'll grant you that there are instance where is "safer" to "not be yourself", like if you're a Queer folk in a conservative environment.
Running back and forth while flapping my hands is not acceptable in today's world. Autistic people need to mask if they want to be successful. It is just the reality we live in.