Well you cannot ever "take" from a dealer, well you can but it is called theft and can land you in a lot of trouble. You have to give, the amount of cash they ask for, and regardless of the customers attitude, loyalty or spend value they should get what they pay for, and maybe a bit extra if they are a good customer, not a whole load less!
In this case some people seem to think they have paid for work / goods that have not been supplied and that is always a big concern with dealers (large and small) I am sure we have all seen this happen on Rogue Traders and Watchdog and I think it is pretty widespread, I have friends in the trade that have worked at places where the bonus scheme rewards shoddy work and corner cutting, the dealer won't admit to this, but often they turn a blind eye when a mechanic is making them a packet by being 200% efficient. This does include oil filters not being replaced (but charged for) tappets / shims not being adjusted (and again labour for work not done is charged) and all sorts of other tricks, my favourite is a mate who worked with a guy that made himself a little tool he could run around the rocker cover that scraped the gasket clean so the more savvy having a quick look would think he had indeed removed this to do the job properly.
Many owners ask for old parts back, but most don't, again I know of cases where mechanics just get the bits from the bin, how do you know they were yours - unless you mark them first.
Are you getting the £40 a litre oil put in? or is it the cheapo crap they buy in 45 gallon drums (assuming they change it at all). Some people provide their own oil, and again I have heard stories of mechanics keeping the good stuff supplied by the punter for their own vehicle and using cheapest crap in the shop.
A lot of dealers have trainee's or "junior technicians" or whatever they call them, but basically you may be paying £100 an hour for some oik who has not got a clue what a torque wrench is to partially strip all of the threads on your pride and joy.
I never mind paying a fair price for a decent job, but I have had bikes where work in the schedule has not been followed (but has been charged for) not only are you being ripped off, but further damage may occur, for exaple tight valve clearances leading to burned out exhaust valves, or FD oil not being changed leading to FD failure - so now you have paid top whack for poor servicing which has resulted in a £1000+ failure.
The problem is you really have no idea how good a job these people do, I am sure most are honest, but I think a relatively large minority are cowboys, same goes for plumbers, electricians, builders, IT "consultants", as soon as you employ someone to do something that you do not know much about yourself, or you cannot see exactly what they are doing it becomes a bit of a lottery.
And why are so many workshops hidden from prying eyes and even if you ask it is not possible to observe your vehicle being worked on, in my job my customers can see what I am doing at any time, if asked a question I will answer it or show them what I am doing and how I do it, why hide if you have nothing to hide?
But if they were all this bad I would expect to see a lot of BMW's breaking down