And if you're a maladjusted immature and arrogant narcissist (I'm not talking about me, btw), should you still be yourself? I don't know. And if you're depressed due to your thinking patterns? Or just rigid and closed, assuming people should adjust to you, not willing to compromise? 

This advice is good, however, for people pleasers. Those should take that to heart. But will just saying that help them? I doubt it since few people do this consciously. Most of them are unaware or try to achieve their own goals by pleasing others - so it suits them, it has its function, although they might not be obvious for a bystander.

Depends on your life purposes, Yvona:)

If they are good priorities and useful for you then it seems fine to let them become yours. Perhaps it would be better to phrase it ”don't uncritically adopt other people's priorities”.

Why should one be oneself? That's a good question. Perhaps it's not always a good thing to be oneself. If for instance Adolf Hitler was himself, it might have been better if he had not been himself. Perhaps it would be better to phrase it ”don't uncritically be something, whether it comes from without or within”.

I wanted to provoke you for some reasons, Yvona.

We will see:)

No, I do not think so. In lifetime, one must meet quite a lot of complexities to deal with. Some are easy to resolve but some are hardship that require pertinent opinions to help you out of the difficulty. Surely, you may have your own idea with no accepting other's opinion and persist on something wrong, but you may as well accept other's opinion to avoid the mistake that will prolong your sufferings. 

Remember being yourself does not indicate you are meant to be headstrong. That is a different notion.