Since anyone can run Arch Testing--it is somewhat similar to Manjaro Testing--I read @jonathon's linked page (did all of you?) for clarification.
As far as Arch goes, the more testers the merrier. And just FYI, it has been my experience that Arch Stable is generally as stable as Manjaro's--for the right user. What Arch is not is Linux for the masses.
Beyond the benefit to Arch Stable users like myself--and, yes, there are many of us that cut their teeth on Manjaro--is the benefit to everyone that uses Manjaro and every Arch-based or based-in-Arch distribution.
The impact felt cross-distribution could be immense. If there is enough participation, though any amount helps.
But that means switching to Arch and being willing to learn The Arch Way. That includes following the Arch Installation Guide and not some YouTube video or install script. None of those other methods--or distros--are supported. So if you do sign up, please be aware of these very important facets.
For myself, I'm perfectly happy having recently got (Arch) Stable all figured out on this new desktop machine--it's different from the laptop I've been running Arch or Manjaro on for the past few years.
But I do intend to follow the process. Thank you, @jonathon.