Re: What is systemd
"Why would they do that rather than running Debian Jessie (without systemd)"
Good idea. What's needed is a version of Debian Jessie in which without systemd as the default.
Why do you care whether it's the default?
Seriously, why do you care? It stil works with either of the alternatives.
Here is the list of packages that depend on systemd in current Jessie:
$ apt-cache --no-breaks --no-suggests --no-recommends rdepends systemd
That's an overestimate because apt-cache show doesn't always put the "|" if the package you're looking for is an alternative, for example lighttpd depends on "lsb-base|systemd".
If you check each package (ignoring the systemd ones, of course) you'll find that the only package in Debian Jessie (or Sid) that depends on systemd is gummiboot. Some tentacles, eh?