I honestly hate it. It adds tedium to an otherwise casual and fun little handheld serious for seemingly no real good reason. I'd understand if it was something more survival based, like Don't Starve, but... it just doesn't feel like it belongs here. It almost made me give up on Blue Rescue Team, though I DID stick through it.
Dark Cloud (non-Pokemon) is another game that has a similar concept; thirst. It's the first game I ever played with a meter anything like this, and... I hated it then, too. I adore Dark Cloud, but that really, REALLY hurts the experience.
Sorry to anyone who likes these kinds of mechanics... I just don't. Like... at all. They add nothing but tedium and frustration for me.