The problem is that if you're only doing 1 group at a time, you might as well drop the healer for a 3rd DPS. The incoming dmg is too low to justify the healer. I can confirm firsthand that expert can be done with 1 tank + 3 DPS.
Large pulls help DPS push large numbers, they get more bang for their buck when AoEing and using dmg cooldowns. Healers can recover MP between pulls, so long as they don't do something silly like Regen you while pulling big. When you do pull big, ensure the healer isn't overstrained via defensive cooldowns.
