expand_less Not all classes are created equal.  Even in the hands of new players unfamiliar with their potential, the core classes can feel unbalanced.  Over the years, the community has tried to explain this state of affairs with the Tier System for Classes.  Simply put, it is our best approximation for the relative power and versatility of each class.  This is useful when considering gestalt because gestalt tends to magnify the power and versatility of each class.  The gap between a tier 1 class and a tier 3 class becomes even wider in gestalt, especially if one character is a gestalt of two tier 1 classes.  To improve the viability of lower tier classes and the character concepts they support, I turned to the tier rankings to balance gestalt builds.
Continuing the example from the "Why Gestalt?" answer, an investigator|ninjaInvestigator|Ninja gestalt character might better fit the concept of Batman than a pure wizard,Wizard, but a pure wizardWizard still has the potential to be more powerful and versatile than a mostly mundane character, especially if they gestalt with another class.  We don't want players to feel that they are missing out by choosing less-optimal options, so we give incentives to players to pick combinations of lower-tier classes.  In principle, this means that both a single-class wizardWizard and an investigator|fighter|monk|ninja|rogue|vigilanteInvestigator|Fighter|Monk|Ninja|Rogue|Vigilante would eventually measure up to the same overall power and versatility.  In practice, this depends entirely on the system mastery and skill of the player.
While tiered gestalt dramatically raises the ceiling for character optimization, it also increases the floor in a way that provides a better foundation for those that can handle its complexity.  The tier system will always only be a rough estimate for each class, but it's the best we have for now.  I'll continue to re-evaluate the base tier for each class, and consider what options are sufficient to change the tier of a class.