Fantasy football managers often put a lot of thought into the offensive skill position players on their rosters. Defenses don't get quite as much attention, but they are often what makes or breaks a team's chances of winning any given week.
It's hard to predict how a collective defensive unit will perform week-to-week. Sure, the league's best and worst units are often consistent, but sometimes great streaming opportunities are created quickly by injuries to key offensive players.
Of course, it's equally important to make sure your DST unit doesn't have any key injuries among its starters. Sometimes, all it takes is one injury to a key start to dismantle a quality stop unit.