Resources let's you buy things, mercenaries sell their services.
If you want to hire mercenaries, you need connections. You can't just hop down to the local store.
And besides, hiring people is a pretty involved process. Buying is comparatively easy. That's why it makes sense for Buying to be a Resources trapping (and therefore doable with one roll) while Hiring isn't.
Besides, it's good for game balance to have acquiring minions be difficult.
The detailed summoning rules are great for building a golem or something like it, a magical robot, if you will.
Funny you should say that...IIRC they were originally intended for golem creation. But they were extended to cover all minion-making because, as you said, it's best to "look at a ritual from the result end, not the means end".
But even there, I would probably just make it a craftsmanship replacement or something similar.
No amount of Craftsmanship will let you create anything as functional as your average golem. It's just not technologically possible.
PS: The system in question here is actually designed to accommodate a zombie's need for drums and the likelihood that a demon will turn on you. You can build complexity by having your summon have some kind of defect that'll eventually lead to a free Compel.
