You get a buff provided the group is in combat with the boss for a certain amount of time. No insta-wiping to stack the buff. It resets after every boss defeat, and doesn't appear for trash.
LFR in MoP, while not impossible, is a fair bit more difficult than in Cataclysm. In Cataclysm, yeah, it was relatively rare I was in a group that died to any boss that wasn't Deathwing, and that was either due to a lack of DPS or some douche deciding it'd be eternally hilarious to pull DW every time everyone sat down to get ready for the boss fight.
In MoP the trash itself can be fairly deadly, and unless everyone in the raid actually knows exactly what the fight is like, the bosses can take several attempts to defeat. The buff works for that purpose. (Some bosses are kinda cheap, too, Dumuru's maze is difficult to get the hang of the first few times and it usually wipes half a raid almost instantly, leaving the rest to finish the fight off).