Why would I want to register with ORCID? I've noticed that in the ACM article document class you can now list authors' "ORCID" registration key.

I've not heard anyone mention that to me before. They Orcid website says:


  ORCID provides an identifier for individuals to use with their name as they engage in research, scholarship, and innovation activities. We provide open tools that enable transparent and trustworthy connections between researchers, their contributions, and affiliations. We provide this service to help people find information and to simplify reporting and analysis.


Whenever someone tells me they want to "enable trust" or that they're "trustworthy" I get suspicious...

So, is this about disambiguating researchers with the same first and last name without relying on their affiliation? Or is there something else to ORCID?

More specifically - if my name is rare and I don't expect there to be another academic with this same name in the foreseeable future - are there benefits for me in registering?
The advantages are:


Unique identifiability (as you noted, not everybody needs this).
Stability: There may be some other researcher with your name in ten year from now. Or you might change your name due to whatever reasons.
It's handy: Many editorial system allow you to log-in by just clicking "log in with ORCID". Some years ago I had zillions of log-in for different systems, often conflicting and I ended up requesting passwords frequently, delaying the submission of reviews. Now I use ORCID for everything and the problem is basically gone (sometimes I get invitations to review for journals I did not use before and they give a new log-in but I insist that they use my ORCID, and this works). For example several sites with the url editorialmanager.com allow you to sign in via ORCID. If you do so you do not a new log in and on top of that all your affiliation is already filled in after log in.
Your university library may be happy: When you use ORCID it is pretty easy for them to see how you publish with different journals/publishers and this may give them useful data for negotiations with publishers. You may have heard of the "Project Deal" where Germany is fighting with some publishers and it turned out that the data about the publications habits of German scholars is helpful in the negotiations (at least this is what our librarian told us).


Actually, I don't know of any competing standard and I do not see any downside in using ORCID. The organization behind ORCID is non-profit. I view ORCID as trustworthy and helpful as the arXiv (in case this means anything to somebody…).