I'll open with a quote from Azuriel on the topic:
People that had no problem being terrible raiders, or otherwise expecting the guild to provide them with endless dungeon runs so that they could guild-hop/get poached three days later.This sounds kind of familiar, a situation in which a group did something for one person, and the following day they disappeared...there was a buzz about it several months ago when a player's guild got him Dragonwrath, and the following day he signed up with a higher tier guild.
The problem is that attunements, like legendaries, require a group collaboration to complete but end up benefiting a single person who has no real reason to remain with that group afterwards.
Also, Attunements are more or less gone and I don't expect we'll see them again but they did serve some purpose. They provided context for your character's effort that was beyond "Show up, get loot"...which while still true, at least now you were showing up to Onyxia's lair for some in-character reason. They provided some entry requirements which are arguably a benefit.
So instead of further debating how vital or important they are, I thought instead I'd examine some possible solutions. I can't claim I was the first to think of any of these but I think they would help.
In this situation once a guild completes an attunement any member (or with a certain reputation level) becomes capable of entering said instance. This way the guild benefits from the activity of its members. On the other hand, it means people who have never done an attunement might avoid guilds who don't have it, essentially letting others do the attunement work for them. While this would benefit high-end guilds who have no problem with the attunements because it would mean they only have to do it once, it also creates a barrier-to-entry for lower-end guilds who might struggle with more difficult attunements. Though once more, provided the attunements are relatively simple it might be a better option.
As for the legendaries, my theory is that they be owned by the guild. For every time the guild completes the quest (and they can only work on one at a time) they gain one legendary to hand out. The Guild Leader can "give" it to one person of his/her choice and they get that in their inventory. If the player leaves the guild, or at any time the leader wants, the item returns to "guild" possession. This ensures that the guild benefits from the work they put in, but it also means once more that freeloaders might try to join guilds that have access to one or more of these items and ignore guilds that don't. It also creates an issue where guilds, even high end ones, might feel forced to run old content to get more "copies" to beat tougher content. However on the plus side, having the legendary be mutable might allow guilds to have a little "fun" with it...set internal competitions "Highest spellcaster DPS on fight X gets Dragonwrath for Ragnaros"...which while a potential benefit, can also lead to drama. However, I think a little disagreement about who gets to use Dragonwrath this week is better than the person jumping ship once they get it.
I think the title explains itself, but while it would not stop the issue where a person gets poached the moment they get attuned, it would at least cut down on the number of times a guild has to run someone. I am sure many guilds ran into the situation where they had to ask a member to change characters to a tank/healer, only to have to re-run them through all the requisite attunements so they could fulfill that role...often because their last tank/healer got poached!
I am not sure legendaries should get this treatment...and if they did, only one could be "out" at a time. It is a prickly way to go, since it would mean for some of the more versatile legendaries (like Dragonwrath) once a player had it, their alts would always have access to it. Though that might make them feel more...Legendary, if they can be handed down....I will have to ponder this.
As I sit here I feel like the first, Guild based, is better because it at least rewards the group for a collaborative effort. On the other hand it might exacerbate issues like guilds being sold, and that all the power would be in the guild master, now with the ability to hand out legendaries. On the attunement front, I think it is a fitting solution...if Blizzard really wants to be safe they could require that the person be at least Friendly with the guild to prevent hopping, but on the other hand we don't want people to "lose" access to content just because they switched guilds.