And, anyways, if you had a libcoreutils, suddenly you're stuck worrying about symbol versions, LD_PRELOAD, etc., etc., whereas simply executing the binary is pretty simple.
The performance overhead of the script-heavy init system that preceded it is in fact one of the core design points of systemd. Boot time still matters in some environments, and the old init scripts were completely out of hand.
... was the non-shell-script upstart on at least two major operating systems, for quite a few years.
No this really isn't a primary design goal in systemd at all. In fact, Lennart Poettering even lists that misconception as myth number two on http://0pointer.de/blog/projects/the-biggest-myths.html