You're both right and wrong. Yes your odds of getting engrams are markedly improved by taking the time to kill every last bugger between you and the boss. However, a lot of us, especially at higher levels, are grinding strikes for bounties. I'm working on the Bad Juju bounty and must complete 25 strikes. I REALLY don't care to slaughter every single minion every single time since there are only five strikes to pick from and it rapidly gets uber-boring.