I don't know if interbreeding is a problem but it could be if all your local bes are derived from a few ferals or your own colonies. I just see it this way, say you have 100 colonies, each one putting out drones. There might be 1-2 queens coming out to mate in a week for example. She might mate with 30 of them, and some will be from the sames hives, excluding a minimum of 70% of your colonies from passing on their genes if they were interbreeding just around your hives. Now we can speculate on drone congregation areas etc... where there might be even more drones from who knows how many other local colonies all competing to pass on their dna to the few queens available every week. Which also brings us to mite vs bee interactions... Mites go through a generation a brood cycle, bees, 1-3 years, thus their major disadvantage in competing with mites on an evolutionary scale as well, but drone pressure helps future colonies but not as high as turning a generation every 21 days. I think it also brings us to the point of having a good diverse genetic make up in your hive by having a well mated queen from different stocks.