Well, lets say you had some kind of issue with the pollen filter failing - I can't think what, but lets say whatever is downwind of the filter getting gummed up with crap. Heater stops working - they pull the heater out and find a whole lot of stuff in there. Even if it's not because of a bad filter, or the way it's installed, the fact you put it in would be an 'out' for them- just like any insurance company.

Re. the self servicing issue - yes there are some things you wouldn't bother doing yourself, you could still arrange it. Yes like AC refrigerant, tyre fitting. Most regular stuff is easy. I've done a timing belt kit and that's about as hard as the routine things get (what's in the service manual). There are some things I can't be bothered doing, but it's painful letting the dealer do it - the local one charges ridiculous rates for the simplest things.