I love this debate, always have. The key to this debate is rather simple, it's the third factor of time. Obviously, flying skill and gunnery are both extremely important. As some have alluded to having good flying skill without good gunnery means you must manuever into a sure-shot position, this takes time and often an intrinsic turning advantage plane vs. plane (you will notice alot of good sticks with bad gunnery fly Spit5's or Niki's for this very reason). However, if you have good gunnery without good flying skill you can successfully shoot at even terrible angles, this does not require alot of time, much flying ability or a particular dominating aircraft flight characteristic.
In a 1 vs 1 duelling situation you have all the time in the world to work an opponent into a sure-fire position with flying skill alone and enjoy much success. In the typical MA furball environment you do not have that time typically. Spend even a few seconds on a target and you are in dire danger of getting your arse blown off. It is for this reason, in the MA at least, excellent gunnery is of premier importance.
I'm sure many of you have been in the situation where you know you have an opponent out-skilled but he has a major E advantage affording you only the briefest high deflection snap-shots. Eventually in this situation you can work down his E advantage with flying skill, but this will take alot of time, therefore putting you at great risk from his friends. With great gunnery, however, all you need to do is peg him on one of those brief snap-shot opportunities and it's over.
I don't agree that great gunnery can be taught per se. I think to a certain point practice and experience can improve your gunnery but nowhere near the level of great or even good if it was bad to begin with. Some people just have an innate 'feel' for deflection. AW back in 1991 was the first flight sim I had ever played, right from day 1 I had great gunnery and a great sense of deflection with no practice or experience of any kind. My flying ability sucked as you would imagine but I still enjoyed alot of success even when compared to much more experienced players. My flying improved dramatically with experience and practice over time, my gunnery more or less stayed at the same level even after years and years of experience and practice. I'm sure many can relate the same subjective experience. This phenomena was noted in real life as well, some people were just born good shots, no amount of drogue shooting made the terrible shots anywhere near as good as the innately good shots who never had or needed any practice.
So, it's not that gunnery is more important necessarily than flying skill, both are equally important in the big picture. But, in the typical MA environment great gunnery will provide you with greater and more consistant success than flying ability in the relative absence of good gunnery all other factors being equal.
Zazen