You want to get your Vashj and Kael vials, you want to complete your Mount Hyjal attunement, and you want to complete your Black Temple attunement. If you're in a guild that isn't running those instances anymore and is focusing squarely on the Tier 6 content, then you still want to go back and get the attunements.
I'm glad you asked.
First, it shows that you're dedicated to your character. This demonstration of dedication is important when playing with people who have, in some cases, spent years fine tuning their character and game play. It also shows that you're up to a challenge and are willing to put forth whatever effort is necessary to get the job done. Tier 6 content is the most challenging content the game has to offer – and you want to show that you're ready, willing, and able to meet that challenge.
Secondly, the attunement provides critical pieces of gear. The rings from Mount Hyjal are the best in the game. Some might argue that such and such a ring is better in a certain circumstance for a certain class, and they would be right, but overall you won't find anything better to put on your finger. For instance the tanking ring gives you a proc of 800 additional armor that helps reduce overall damage intake, and thus required healing, over a boss fight so much it's ridiculous. I dare say that it's over powered.
Finally, attunement just looks good. And no, not the title of the "Hand of A'dal" that you get. It looks good on any application to a guild. Tier 6 guilds seem to be constantly recruiting these days, and there's nothing wrong with that. However applicants who have the attunements done stand out above the rest. I can say that when I'm browsing and commenting on my guild's applications – those that have completed the process have just another thing going for them over those that haven't. This might not seem like a big deal, and you might be thinking to yourself, "This doesn't apply to me." And it might not – you might be uber enough that you don't need attunement to stand out. But trust me, applicants like those are few and far between.
So how can you get your attunement? First and foremost, try to stick it out with your current guild and get attuned with them. While it might take everyone a couple months to down Vashj and Kael, the first four bosses of Mount Hyjal and the first four of five of Black Temple will be downed within weeks of finishing off the Tier 5 content. Major loot and success is not far away, trust in your guild and try to stick it out.
However if you can't stay for whatever reason, buy your attunement. Guilds that are farming Vashj and Kael will often times be willing to let a non guild member in for a large amount of gold. Be prepared to pay upwards of 2000g for both the vials. I've seen the price as low as 1500g, and as high as 4000g. But that is the price you have to pay if you want to go this route. Farm, farm, away for a month or so and you'll have enough to buy yourself the attunements.
No matter how you choose to go about the old attunement processes, just remember that they're there for a reason, and that they still do matter. Especially when apping to guilds.
I'm interested to know what you all, our readership, thinks about this. How much weight does your guild put into attunement?