DPR is king as long as the bad guys are not threatening. If they have abilities that screw you up or have heavy attacks, they can focus fire a striker down fairly quickly. A controller make it so that they don't attack at all, a defender can absorb the damage. A leader can do any number of things depending on class. Make it easier for you to hit, make it harder for them to hit, lock them down, etc.
DPR is still king in these situations. A dead foe cannot screw up the PCs.
This is especially true now that monsters do ~1/2 level extra damage per successful attack. The monster effects had a greater overall impact when the monster damage was less. The monster damage was less of a factor. Now that monster damage is greater, it is even more critical to take monsters out quickly than it ever was before. PCs have to mitigate effects and they have to mitigate more damage. Every battle by definition now uses up more party resources on average.
Looked at another way, with the monsters doing more damage, it is more important to do some form of damage mitigation: surgeless healing, killing monsters faster, temporary hit points, damage resistance. Anything to prevent PCs from falling unconscious and shifting action economy into the favor of the monsters.
Yes, a controller can totally lock down a foe. Typically, it is one foe and it is often only for 1 or 2 rounds. Yes, a defender can influence a few foes to attack him. But the fact remains that the foes are still attacking someone.
I've seen defenders use up 5 healing surges in a single encounter. Sure, the striker would have used up 6 or 7 healing surges in the same focus fire situation due to getting hit slightly more often and healing slightly less per surge if the striker did the same amount of damage as the defender. But the striker takes those same foes out faster than the defender. Typically in 2/3rds or less of the time. That means that the striker would really use up about 4 healing surges in that same situation plus the monsters have fewer opportunities to put conditions on the striker since they get fewer attacks.
Defenders don't really mitigate as much monster damage or save as many party resources as people think.
If you are doing 1 or 2 encounters a day, you can probably afford to have strikers take damage. But as the adventuring day winds on, healing surges start to dry up.
Except that a high DPR group (not necessarily all strikers, but PCs that can nova) wipe foes out quicker and hence, their group does take less damage. DPR combined with focused fire mitigates using up resources more than any other game mechanic overall.
THP, surgeless healing, debuffs, and damage resistance all mitigate monster damage to some extent, but not nearly as much as taking foes out in 2/3rds the time.
And there are always exceptions based on situation. The controller who slides a foe into a deep pit, both damaging the foe and preventing it from attacking for 2 rounds until it can stand up and get out. The leader who added +2 to hit and that 10% extra chance to hit allows someone else to hit where they would have otherwise missed. All of these add to the overall party effectiveness. It's just that the +2 to hit only changes the situation 1 attack in 10 when it applies whereas taking a foe out earlier not only typically prevents one attack by that foe, it allows the PCs to focus fire more on the remaining foes and take them out quicker (hence, mitigating monster effects and damage, and using fewer party resources).
And an all damage approach doesn't work either. Parties need healing and handing out of free saving throws and ways to avoid or overcome monster effects. It's not all about damage. But the bottom line is that the one game mechanic that is responsible for the winning of encounters is damage. DPR is king. There really is no good way to Mass Charm a group of monsters and have them walk away.
Delaying attacks is helpful, but preventing them completely is preferable.
