Back when I raided, there was a thing that really annoyed the hell outta me.
There was this guy, whenever he got an upgrade, he'd hearthstone to Orgrimmar to gem, enchant and reforge. Talk about wasting time. Then we'd have to summon him back. And run back to wherever we were. At some point, our Raid Leader threw a fit. "We have an hour left to the raid, you can do all that shit later. For now, keep your old piece and get your ass back here." It worked, but the guy was way too grumpy after that.
That was before. Now that we can have access to a mount which provides a portable reforger, I'm worried this will become the norm. Everyone who gets a new item will want to use the mount to reforge. Of course, no one will have to HS back to Org or SW. Now, they can just mount up and reforge.
But how much time will be wasted on this between pulls, making wipe recoveries last that much longer. Reforging is not like repairing. You don't just click on a button and it's done. Sometimes one piece changes some other pieces' reforging as well. There's always one dude taking his sweet time making the rest of the raid wait. Don't be that guy. Wait for a break, or for the end of the raid.
That said, the Yak really is cool. I don't think I'll buy it simply because I'm too greedy and don't want to spend my whole "fortune" on one mount, but I can't deny it's usefulness. I think it's nice you can reforge in whichever zone you are. If there are no reforgers in Pandaria, then it's awesome to be able to carry one around with you.