This would Sir, this would imply you have a good understanding of "all" systems enough to make this statement. I'm guessing this is not really the case?
True, some do train this way, and its a shame when they feel it makes them safe in reality. Again, you are making statements about training you have not witnessed. 10th Group Special Forces, as only ONE example, trains a system that is rich in just that - multiple assailants with weapons. They have to. I've laid hands with those boys, they don't play around, their intent in a confrontation is the real deal, and they don't let their comrades walk away with something that will not work. So, yes, its multiple weapons at the same time, against big fast dudes who all kick **** in their own right (cage fighters, boxers, kickboxers, spec ops, and the general steel jawed drill seargent types, you name it). And yes, the system does work. One thing is emphasized, though - stay on your feet. The ground is not a good place to be when people want to kill you.
Give me a knife against a chair any time. I'll may take a lump on my arms on the way in, but there'd not be a second swing.
Only in the movies would someone attempt to stand in the middle of an enclosing circle. Again, you jut haven't seen what all is out there. I encourage you to keep looking.
This has been a good topic, I'm interested to hear more. Thanks in advance,
Steven