Only the few people who ask higher than they have are looking for people to "outgear'" the instance.
A person with the Heroic Warforged Frenzied Crystal of Rage can still find an upgrade in Flex. Or a spec like Fury or Fire, which rely super heavily on crit, can find better items in a lower difficulty because of the itemization. Or tanks can pick up the trinket from Flex protectors to use on heroic Malkorok for phase 2, if needed. Nobody is running the instance if they are no longer needing the drops, but just because somebody is at 560 or higher gear level doesnt mean there arent upgrades, especially with the new 4/4 valor upgrades, they can have LFR pieces on and be up at 560ish, so they can benefit from Flex and certainly from normal drops. Item level is so incredibly skewed by the upgrades and Ordos drops even moreso than past world boss patches. Until all your pieces are Warforged and best itemized for your spec, you can still use more from normal, which would go up to about 575 or 576 item level with 4/4 upgrades. At 576, they could use a different itemized warforged piece from normal if is better, or even at that item level they may want to run normal for tier pieces, since 575 would be without tier since there is no warforged tier pieces. 575 for normal mode would NOT be just for those who have all their drops. it would be for people who only have 1 or 2 very minor upgrades left, but nevertheless, its certainly not exclusive to those who have all their drop.
Just because you have below iLvl580 does not mean you like to get carried, but what is the point of doing any kind of raid if the loot you can get is useless?
because you rarely do raid where everything is useless?
If you have full LFR gear you have already ilvl of 550 (with legend cloak and upgrades). If you than go into flex you still can have upgrades there
reference table, there are 16 slots of gear with ilvl (15 if you have 2H wep). the cloak have 616 ilvl.
with +16 upgrades you can have fullset of
LFR at 548ilvl
Flex at 560ilvl
Norm at 572ilvl
HC at 584ilvl
Norm and HC is without Warforged, with them you can add +4 ilvl to both
going flex with 540 have still upgrades for you.