The big thing with leasing privately is even though it is more expensive the Motability, you can have any car you want (providing you can afford it of course).
However lease schemes tend to have a fairly high initial payments as well, you have then got to factor in insurance, tax(depending on vehicle) as well as the mileage limititations and thats before factoring in any adaptions needed or if indeed the lease company will allow them to be fitted.
Of course for many MB users it could make perfect sense to lease privately at the moment, for us with all the adaptions we require leasing from MB is still the better option.
You pay your monies and takes your choice I suppose.