On my existing A3 I've only fitted 2 new tyres in the 36k miles from new, but I agree with you that I want to retain the decision about what tyres and when I change them and who fits them. My new Sportback has Michelin tyres fitted.
I'd certainly be quids in as I'm going through a set of fronts every year,at a cost of approx £210 per pair (fitted).
I bet you my house they'd throw their toys out of the pram and increase my monthly costs though.
My A4 was leased on a maintenance deal,but I didn't like the fact they told me which tyre shop to use,then the tyre shop told me they could only fit certain tyres.
It got silly.
I ended up with 4 different tyres on each wheel,even if they were the same actual brand.