I have been in Avalon for a very long time indeed, and in all that time teaming has ALWAYS been a way of life.
No-one city is more or less reponsible for it, it just happens, always has, always will.
So why waste time arguing back and forth about it? You know what happens, a group of X people from Y city jump and team and strip you, so the next opportunity you get, you do the same.
I do not think that there is a single person from any city that has been in the land for any length of time that has never been involved in a fight where they had superior numbers on their side.
As to challenges and its value being lost, well they NEVER have been used in an honourable way.
They have always been used as either, 1) an excuse to attack whoever you please, 2) to lure people out of 'safe' locations, 3) to give a chance to pour derision upon someone who fails to accept a challenge and 4) to cause as much 'damage' to a target
There never has been much honour in the land, and those that try and stand by it have never in my experienced managed to maintain a consistent standard of honour.
Life's a bitch, then you die!
Written by my hand on the 20th of Midwinter, in the year 953.