As a player, too much metagaming can be very destructive to a campaign.  Metagaming takes you out of the mind of your character.  You begin to make decisions based on game mechanics and plot predictions rather than in-character motivations.  Often, this either leads to or is driven by a desire to "win" in some way.  This can quickly derail a campaign, turning it into a "who's the coolest" competition among the players.  As a result of all of this, metagaming is often a dirty word within a gaming group, connotative of min-maxing and munchkining.

As a GM, though, this analysis really doesn't apply.  As a GM, your job is vastly different to that of your players.  You are not spending your time embodying a single character, trying to realize that persona as completely as possible.  Instead, you are guiding a story, flitting from character to character as the scenes and seasons change.  You have to balance the needs to the various NPCs with the needs of the story, keeping everything on track without making the players feel railroaded.  You need to take a completely different approach to the game... and that approach is metagaming.

Consider this situation:  A team of PCs is meeting an important NPC for the first time.  He is a wealthy and powerful man, far above the level of the PCs (either literally or figuratively); more importantly, he is integral to the plot of the main storyline.  The PCs recognize the situation; they realize they should show proper deference to El Hefe (for whatever story-relevant reason).  However, a player, playing his character fully in character, says or does something to offend Biggens McCheese.  How should the NPCs (Mr. Honcho and his possible cadre of guards) react to this?

Playing the NPCs fully in character, it would not be unreasonable for the situation to quickly go south.  The PCs could be ejected from the premises or even killed.  But is this a reasonable course of action for the GM to take?  I think the answer is a hard maybe, and to really address the scenario, the GM needs to rely on a metagaming analysis.  The GM needs to consider:

• Is a harsh punishment for the offending PC appropriate for the campaign style and the situation?
• How would the player(s) react to the death of a PC?
• Is there another way to bring the PCs back to the storyline, or will the loss of this NPC completely derail the campaign?
• As that goes, is the GM willing to delay or abandon that storyline and work on another, adjusting on the fly?  That is, do you as a GM have something else prepared, or are you very comfortable with improv?

Now, don't get me wrong:  I'm not suggesting that the NPCs should behave like idiots or bow to the whims of the PCs.  I'm only suggesting that, as a GM, you need to consider an array of reasonable, e.g., reactions that the NPCs could have to the PCs behavior.  You then need to keep the broader goals of the storyline in mind when you decide what happens next.  Given the above scenario, perhaps the NPC is willing to overlook (however temporarily) the offense given by the PC, allowing the PC to either earn forgiveness or be punished later; or perhaps he's willing to demonstrate his displeasure in other ways (monetarily, for example).