It's more of a situational thing.
Milkruns, or any other sort of strategical/tactical "dweebery" themselves are almost impossible when arena numbers are equal. Milkruns aren't very likely when numbers are balanced, and since every country is in a simular situation, things like dive bombing Lancs, or massive Typhoon raids are usually less of a threat.
However, once the numbers balance shifts to one side, milk-running, dive bombing Lancasters, massive Typhoon raids become a serious threat which the receiving end has no possible way to counter.
While every pilot is tied to defending important areas in the lesser team, the larger team has a significant reserve of pilots who can be organized separately without having to risk faltering defenses. Thus they merrily go swarm an undefended field with multiple planes, and there's absolutely nothing the defenders can do about it. Alternative defense tactics such as temporary point defense, or goon hunting, fuel porking can maybe delay enemy onslaught for 15 minutes ~ 1 hour, but that's about all it can do.
So what happens is, the pilots of the defending side who are already facing harsh odds, vicious and brutal battle near their position, get to see zillions of enemy dots moving to a friendly field at some backward, undefended sector, and grit their teeth in anger that they can't do anything to stop it.
So in a dilemma, whether they should give up the most important strategic position which they are defending, and deploy pilots to defend a less important backward sector, or keep defending the position while the enemy swarms to milk-run every existing base in undefended sectors... people get frustrated, real frustrated.
