I really like these as they are presented. They're mostly designed for smaller groups, but also work well for drop in/drop out NPCs. I can see this working for a special mount, beast companion, or familiar at a cost....maybe a special quest!
This would also work as a very tangible reward for exceptional role-playing for those using milestone XP.
That was one of my first thoughts: "Where does this fit?"
It was mentioned there would be a campaign setting released next year, what setting does this work best in? The first one that comes to mind is Dark Sun. Between the half-elf's animal companion, all of the followers PCs could amass at later levels (if they decide to go that route these days), it could also be an alternative to the old Character Trees. It could also work for Birthright (wishful thinking) with ALL of the retainers. Spelljammer could also benefit from this. A PC is the captain of a ship and leaves his sidekick in charge when the party disembarks to do PC-type things.
It doesn't even have to be Spelljammer. With the last UA covering ship stuff, this could roll right into that. Or something else entirely. The PCs have their Castle of Greatness (TM and Patent Pending), they leave their sidekicks in charge while they run out to topple the big bad. The big bad launches a surprise attack on the castle and the PCs now take control of their sidekicks.