Our current GM may have a fit if we introduce any sidekicks, as we have an especially large group for the next campaign. On the other hand, the majority of campaigns I've played in or DM'd have, at some point, seen a player or the party as a whole 'adopting' some creature or person, so I could definitely see myself getting some use out of these rules.
It occurs to me that they might also be a handy way of creating simplified NPCs, for when you want something a little more substantial than the Monster Manual stat block, but don't need to go through an entire class and subclass progression. You could also use them to beef up any monster or NPC if it doesn't quite measure up to the CR you're looking for.