Not only would this feature be useful and entertaining for raid leading purposes, but for anyone to be allowed to observe any player doing any activity within WoW. This has a countless number of uses. Whether you're teaching somebody how to beat Captain Skyshatter or simply just wanting to see how overpowered ret paladins really are.

Recruitment Made Easier

An example of this feature, is that an officer can use the Observer Mode to closely observe a potential recruit's play. Observation of a trial member for a guild is far more limited than it needs to be. In numerous other games, there is a way to observe others playing closely. Any serious ex-Warcraft 3 player knows of downloading replays and being able to observe things from a player's PoV, or simply in custom matches players can become an observer instead of playing themselves. Nearly all FPS games offer this feature, and not only does it have its uses for gaining further knowledge of the game (stealing their tricks), but recruitment for guilds/clans would be drastically harder without the feature.

In the Raid

For many raids, if the raid leader doesn't show up to a difficult raid, most members are going to want to /wrist. A raid leader brings much more to a raid than simply his avatar. But, his character must take one of the 25 slots available to large raids every week. Not to mention as you're reading this, he's probably burnt out from having to attend and play in every raid. What if your raid leader could lead the raid, without taking up a spot in the raid and not be potentially distracted from him having to play his own character?