A problem is the stupid times you go in there to deal with Cabal. It's made clear that the Vex are simulating a situation where the Cabal are escorting important information. They you get "we can't let the Cabal leave the simulation with whatever they found."
Bungie stopped caring about accuracy in their story telling for some time now.
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So the original mission in the main story quest in the tree of probabilities is the cabal trying to get information, they aren’t a simulation, that parts easy. But in the strike, we know that the vex are running a combat loop, of the fight that just happened, and Sagira even says the cabal will lose everytime. Like you said, it begins to blur because sometime at the beginning of the strike Sagira will say that we can’t allow the cabal to escape, even though they supposedly never will and will always lose. The cabal lost largely because of us though. If we weren’t there looking for the map the same time they were, they likely would’ve gotten it. So, the cabal will always lose as long as we are there. So whenever the vex run this simulation, guardians need to go in and stop them or the vex will get further informations that might lead to a dark future.
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To make their Cabal dude have information to carry, they have to make the information. That's my problem with the story logic here. Think of it this way. You make a game where you have to forces fighting eachother. The idea is to stop the game when you either acquire the information from the enemy force (you had to program that information into the game for it to have), or... an entity comes into the game to stop you from getting the information from an object you already created... Basically, why the hell would I chase down a programmed object that I made, for the information that I made, and assigned to the object.