Yea, plateaus have a purpose. Your body knows what it's doing.
Here's a link that explains it a little better than I can.
http://www.weightlossforall.com/weight-loss-plateau.htm (hope the link is okay to post)
Plateaus are frustrating, yes. But if you continue to do the right things, your weight will start moving again. Your body is basically saying, "Whoa, hang on, I need time to adjust to this new weight." It just sort of puts things "on hold" for a while.
On the flipside, yes, I'd much rather maintain than gain. But when we're trying to lose weight and busting our butts exercising and eat the right amount of calories, we want to see that scale move DOWN. I know, I've been there more times than I can count. And when the scale stops and stays there for about 3 weeks (they say it can't really be considered a true plateau until you've been stalled for 2-3 weeks at least) you start ripping your hair out because you can't figure out what you're doing wrong. Do you up your calories or cut them back even further? Do you exercise more? Or are you exercising too much? Blah blah blah. All kinds of stuff goes through your brain.
Weeeee! Been on that trip more times than I care to admit
I'd say you can consider yourself in 'plateau mode' if you're going on week 3 with no loss. After one week? That's nothin. Lots of people don't budge in one week. Two weeks? Okay, bordering on frustrating, but it's still nothing to worry about. Three weeks? Yep, you can start throwing tantrums, lol.