Getting into a Mythic Jaina pug without already having the achievement was proving to be a pain, so I just bought a boost for 400k.
Probably about 1/4th of the gold I was sitting on at the time, but totally worth it to me given the mount will no doubt be very annoying to grind in SL and even beyond. And I have no doubt in my mind that the mount will sell for way more on the BMAH as well.
In that regards, boosts are a necessary evil in my opinion. But given how much the trade channel is spammed, especially on some of the larger realms, I always move trade chat to a separate window that I never open unless I'm specifically looking to buy a boost for something.
And boosts frankly aren't anything new. They've always been around in some shape or form, but I believe they gained prominence in MoP as people wanted the mount from Garrosh Ahead of the Curve and his heirlooms that became guaranteed drops in the prepatch of WoD and would be forever unavailable once WoD launched.