A certification, any kind of certification, is worth more when you are inexperienced and worth less and less as you gain more experience.
When you are inexperienced, the knowledge you acquire from the certification is worth a lot because you might not have had exposure to it and you learn about it as you get certified. If you are a scrum master with 15 years of experience you probably know a lot more than the knowledge required to pass the certification exam, so the information needed to get certified isn't worth as much because it's stuff you already know, it isn't an investment in something new. Either way though, if you are after the information and the knowledge, you will spend the time to acquire it and it is irrelevant if you get certified or not. So, to you, getting certified is, from this point of view, kind of irrelevant. Sure, passing the actual exam is a validation for you that you understood the information, but you could use test exams to verify the same, without actually spending money on the piece of paper that says you know Scrum.
The question is thus only about that piece of paper that says that you are accredited. It's not about the knowledge, since as a Scrum master you still need to know the stuff that's in there.
The certification becomes important for companies as a selection criteria for new Scrum masters. Is it important all new hires are certified or not? If someone has 15 years of experience and great successes helping Scrum teams, but no certification, is that important? What if someone has 15 years of experience and great successes helping Scrum teams, and a certification, is that even more important?
That's why I said in the beginning that a certification is worth more when you are inexperienced. Because, when companies decide to hire junior Scrum masters for example, and they find a few candidates with similar knowledge and skills, and similar salary demands, the certification might be the decision factor since it might show that candidates that have it are more commited to the Scrum practices and more invested into becoming better Scrum masters. But that's a belief, not a fact. As others have pointed out, in this post and in the ones you linked, there is no proof that a certification will make you better at your job. 15 years of experience though, might say more about it.
I used to interview a lot of Java developers back in the days where everyone was chasing the SCJP certification, and what I noticed was that the worst candidates were the ones with certifications. I'm not saying I attributed this as the cause, what I'm saying is that the developers that were bad, were bad even after being certified as Java programmers. If you suck at your job, be it Java developer, Scrum master or watever, you will suck with having a certification also. The certification just shows you can study some material then pass an exam. You get more skills from experience and exposure, not exams.
As the other answer here mentions, you need to search within your job market to see if employers are requiring the certificate or not, and if you want to work for them or not, because not having a certification will be an elimination criteria and a disadvantage to you. But then again, if research show no remarkable difference between those with certifications and those without, and from experience people discover (as I did) that they don't make someone suck less, then why are the certifications so important for such companies? Just saying...
