Flipping on Shadowmeld has its disadvantages (almost any action takes you out of stealth and puts you back into combat), but if you need to duck out of combat for a quick drink or a quick change of gear, you can do it. And it can easily buy you some extra time to summon a mount or get away from trouble if you happen to be in PvP outside. And while it's not quite as powerful as Vanish, you can even use it in the Arenas to drop out of combat (hidden, as when you start drinking you'll lose your stealth), and grab a few seconds of drinking before you get brought back into the fight.
None of these uses are exactly gamebreaking -- the Shadowmeld action is pretty fragile, so you're going to have to be fast if you want to put that second or half second of dropping out of combat to good use. But when time is short and you've got to use every little ability you have, it's important to have Shadowmeld around. Ducking out of combat for just a moment might be just the boost you need.