You are right once the AP gets high there is little advantage to paying more for a higher specification/brand via the scheme. With these cars and the limited mileage most scheme members put on their cars and the condition they are returned in, after 3 years the cars are worth quite a lot more than 1/3 so MB are gaining even more.
Similarly with some of the entry level cars, there are often very good deals on offer at the dealers most of which have at least a 3 year warranty, free roadside assistance and as you have found you can build in a service plan.
There is a middle ground where you can get a very good deal through MB for little outlay but it is getting harder.