The only problem I can see with that is, what about when you're on Mars, and for you the vex have completely taken it over (the cabal are on Venus or something), another player that that hasn't happened to shows up, and cabal starts spawning even though they left.
(Sorry if that didn't make any sense. I really didn't know how to word it.)
English
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Outside of the story it would be the same for everyone. The actions and inactions of all guardians would affect the game world as a whole. The forums could be used to convince and motivate players to focus their attentions on one target.