Using the hive-mind, all Spectres can communicate with each other telepathically, using this ability to coordinate attacks and send out distress calls when overwhelmed by an enemy. It also serves as an information network; with concentration and the spending of Spite, a Spectre can access any piece of information known to any other Spectre, including Horrors.
While it would appear to make the Spectres nigh unstoppable, the hive-mind became a major weakness when Grandmother and the Malfeans went to war against each other. Any attack or battle plan they came up with went into the common hive-mind and became known to the other. Memory Towers, where secret information could be stored, was the only way to assure the other side did not know what was coming. However, nothing stopped Spectres from accessing the hive-mind to find the location of those towers and attempt to penetrate them to get the secrets within.
The hive-mind also had a few weaknesses. Orphan-Grinders have an inherent ability to tap into them, left over from their own Spectral existences, and could use them to access information the Spectres wanted left unknown to ghosts. In addition, some Spectres, such as Mr. Jigsaw, could fool the hive-mind under a number of personalities.