Board Thread:Questions and Answers Forum/@comment-108.12.181.115-20150318203657/@comment-25819007-20150318205502

Light level only matters when you're fighting monsters and they're at a higher level than you. In that situation there's a large penalty to damage dealt by the low light player and a large bonus to damage dealt by the high light monster. If the player's light level is equal or better than the monster, the penalty and bonus are both zero and then the stats matter. If you're at level 20 and fighting a monster at level 25 then it's going to destroy you. If you're at 25 fighting a 25 then light doesn't matter and it's much easier. If you're at 32 fighting a 25 then light still doesn't matter and you have more defense than you did at 25 so you take less damage, but not that much less (compared to how much it went down when you advanced from 20 to 25).

None of this matters in Crucible since light, attack, and defense levels are ignored. In Iron Banner it does matter but not as strongly as it does for monsters in PVE.