Seems like this is the commit causing this: http://cgit.freedesktop.org/systemd/systemd/commit/?id=74397

Why not just sleep for a fixed amount of time?

Because sleep is arbitrary, ugly, and slow?

At least that is one of the arguments i have seen bandied about for going systemd in the first place, a reduction in the amount of sleeps they had to put into the bootup scripts.

Because kill -9'ing half the system is better?

Not defending them, as i find the whole systemd concept a massive train wreck. Just speculating what their justification is likely to be.