TL;DR Answer for those who don't want to read the text below: Damage
The why: Rate Of Fire is a interesting thing. There is something to be said for simply throwing up a wall of Dakka and a hellstorm of bullets, forcing your opponents to either duck away and take cover in a easy-to-kill spot with a well placed grenade or a ally sniper, but while anyone who walks into the wall could become swisscheese, why bother with wasting all that ammo if you could have a gun that turns your target into chunky salsa with ONE shot from a long distance off?
It's safer, more ammo-effective, and most importantly, will FORCE you to aim and be precise with your shots. If you want to get kills, you need to AIM.
Should everyone have these weapons? Of course not, that would be silly, it would be musket warfare all over again. There need to be a balance of users.
But I prefer damage over ROF. Give me a Browning 50. Cal with a bolt-action or a Winchester Lever-Action any day.