Everyone that can go through all the farm needed to finish the quest will probably gain a Legendary.
What is apparent is that the legendary questline is a really powerful motivator for everyone to stay suscribed.
Until now, when a player thought he finished the content (whatever that means for him), he would unsuscribe until next patch.
The legendary quest introduced a farm that garantees that you got a really uber item at the end.
If you remove carrot, especially at the end of the expansion, by gating the legendary to be obtainable only by normal/heroic raiders, you ruin LFR people sense of progression.
It will be hard, it will be long, it will probably be pointless except for the feeling of completion, but it will be obtainable by everyone with a ton of effort.