Well, this is definitely not a behavior I'd encourage, but from their point of view, the idea of controlling EVERYTHING that happens at boot time might look appealing, and they probably realise that they'll have to fight in order to force people to comply with their API. The simplest way they have to get around it is to rewrite everything themselve, so they can implement everything they want, however they want. This is a big task, but they seem to prefer quantity over quality, so this decision makes sense for them.
Now I don't think systemd-mount brings any new feature that would justify this...