(Just a med student but...)
Analytically, it sounds like a do-it-yourself capitation scheme without insurance middleman.
When HMO's underwrite capitation contract, they contract physicians on a per-member-per-month payment basis in return for a basket of services. If members underutilize, you earn. If members overutilize, you lose. As a result, insurance companies need to be real careful as to risk category of who enrolls in these services. It's a good idea for young people (healthy) and a bad idea for old people (use a lot of med services). Insurance companies can get around this by diversifying their patient base by contracting with companies and enrolling many members en masse. This diversifies their risk to an "expected/predictable" level and are able to track that experience over time in their databases (this is the job of the actuary).
So... when your small group practice tries to price a patient (for example) for a boutique/retainer, you are taking on the job of an actuary/insurance company. You are assuming that you have enough information to predict utilization rate of that one patient AND how much those services are worth. How do you know that $1000 is the right price? How do you know that you're not enrolling 400 healthy patients and not 400 sick patients... remember, you are providing all of their medical care for a fixed price (that is capitation). You need to be confident that you can enroll a lot of patients to diversify your risk and you better have a good computerized tracking system that can tell you how you're doing and when you need to change your rates. However, you may have problems enrolling that many patients, so you might have to compete with insurance companies based on price.
If you look at it from a patient's point of view, why would a patient pay you a fixed price for services if he knew that he only sees you once a year. Obviously the people who clearly benefit from these arrangements are those with a crapload of medical problems. From their point of view, something like this would be more economical than paying the insurance company, provided that you quote them a rate that they can't refuse to switch. So then you end up getting sick patients for a lower price... we call this "adverse selection". And once sick patients know that you're on retainer, the smart ones will call you all the time and use you as much as possible... "moral hazard".
It seems to make sense only if you already have experience with the patients you enroll AND are able to predict their future utilization rates. It also only makes sense if you're in a market where health insurance offered to employees/retirees are unreasonable based on those patient's utilization. Personally, I would leave the business of underwriting insurance to those who do it professionally. But I'm only a med student and I'm almost always wrong.