This 'confessions' article about today's use of influencers in social advertising has some interesting points to make, many of them showing no love lost.
Of course, there is a benefit of using an influencer's following to reach a wider audience than a brand might achieve through owned channels alone. However, as the article suggests, the current model is, well, more than just messy… perhaps non-existent, with fees often wildly exaggerated and not anywhere near in direct proportion to their following. They charge what they want.
This is normal, I suppose. It's a new field with little precedent, so pricing and structure is yet to develop. As said, the benefit these people can offer is undeniable, so it will be interesting to see how this new profession develops.