this will contain spoilers about the quest that rewards the balloon, so if you'd like to discover the quest for yourself, don't read on.
after doing the balloon quest a couple of times on the PTR (alliance only so far though), i've come to a personal conclusion that it's actually quite a very sad and bittersweet quest.
perhaps i'm reading a bit too far into it, but hearing vin excitedly say that she can see everything from up high in the air and then an unexpected "goodbye" from her... well, it just left a slight pang in my chest.
up, up here we go
if you read through the quest, you'll note that she says her father bought her the never-deflating balloons from the darkmoon faire. however, where is her father? or her mother for that matter? her proximity to the stormwind orphanage makes me wonder if they are no longer around and if she's one of the many orphan children running around the area. that, and her sudden farewell into the sky... well, you can see how one could extract from this one little quest a sad outlook.
thankfully, though, if you zone or log out/log back in, vin will respawn in the same location in which you initially found her. so perhaps it's not a goodbye forever. or maybe developers forgot to add in a small phase change so that players who completed the quest wouldn't see her anymore.
in any case, the quest is fairly low level, making the balloon an easy pet to obtain for even the newest of players. it's also a cute, sweet quest, even somewhat bittersweet. whether or not it was intended to be, though, i don't know. i highly recommend this quest so you can decide for yourself! :)