The current state of raiding has created a situation where as long as you are in a top 500 guild, you now have access to the best gear and can somewhat compete with the players in the best guilds as far as ranked parses go. Naturally, the top guilds will have players geared in full best in slot gear, while you might miss some pieces due to less kills per boss. They will also kill bosses faster, but on the whole, you have a shot at not being terribly behind and you can get a good rank (considering your gear) even if you don't have best in slot gear. It's rewarding to me to play well and get that rank, and it also lets me know that I'm not doing anything terribly wrong in the encounter. But more than anything, you can legitimately compete within your own guild.
If this legendary is more than just a vanity item, every caster that doesn't have one will be at a severe disadvantage, and I think that in a raiding environment that is so much down to individual skill, no player should have that large of an innate advantage over another player. It's unfair, and frankly, bad design. I honestly can't think of a single good thing coming from introducing items that are a) significantly stronger than most other items in the same tier and b) only available to a few players.
I don't think the raiding community even wants these items because of the uneven playing field they create, so I'm curious as to why Blizzard keeps making them.
PS.
People don't seem to be very good at grasping what I'm saying so I'll make a sports analogy. Imagine that you are on a football (soccer) team. There are 11 players on the field. One player is almost always there, the goalkeeper (the main tank). The other 10 players are mostly the same every game, but depending on what shape people are in they can get a spot over someone else, and you can break into the team by being a good player. However, giving one player a legendary is like giving one player on the team super powers to shoot and run harder than everyone else. Yes, it might help your team win, but there's literally nothing the other players can do to reach the level of the guy with the super powers, and I think that's unfair.