My policy, at this point, is to shoot ahead if the other 2 do also. I understand why people do so as it is tough to get in 25 strikes. But the few times I get to fight the waves, I manage to get some blue engrams, which are useful for upgrading the cryptarch. What is bugging me are folks who abandon strikes. Mind you: your ghost does say at some point in the story, "if you want to get in the middle of this, it's up to you." That is the 1st clue these skirmishes are optional.