I could see distros like arch, that want to test new stuff and are known to work on bleeding edge software would make sense.
That's a bit disingenuous, Fedora 15 (released May 2011) was the first distribution to enable systemd by default, so there were at least 2 years of "rollout and experience" when Debian 8 (April 2015) and CentOS 7 (July 2014) switched. And at least in the case of Debian, it was already available as an option since 2012.
And yes, while available in debian, very few people used it for the reasons I'm talking about: it hasn't been heavily tested.
Debian ramming systemd into stable like they did on the other hand...
This is what lead to Gentoo effectively forking udev into eudev, so that they could go on offering the distro users a choice in inits.
This because the process for extracting udev from the larger systemd code blob would change with every release, and thus requiring manual intervention.
In essence, the systemd devs are playing dirty. And because they are making the life of some other devs easier by offering tasty APIs, the project keeps agglomerating power like some kind of shoggoth.
I am damn sure that once kdbus comes into play, there will be moves made to make systemd the sole arbiter between kernel and userspace. Effectively enveloping the kernel.