My gaming experience largely predates modern massively multiplayer online (MMO) roleplaying games (RPGs), but the design decisions going into such games mimic the design decisions going into their pen-and-paper predecessors. John Hopson looks at class design through an economic lens:
At the heart of the hybrid problem is the fact that if a hybrid can perform a given role as well as a specialist while also having other abilities the specialist can never have, playing a specialist becomes pointless.
To put it in terms of our earlier example, if a paladin can tank as well as a knight but can also heal, then there is never a reason to play a knight instead of a paladin. If the hybrid has all of the advantages of its parents plus extras, then the parent class is doomed to extinction.
Conversely, if a hybrid is always inferior to a specialist in any given role, then it’s always better to have a specialist fill that role. As game designers, we want to create a vibrant ecology of classes, where players have a wide variety of classes and play styles available to them.
The standard solution to this problem can be summed up in the phrase “Jack of all trades, master of none”. Hybrids are generally made less effective in each area than their parent classes, with the intent that they make up the deficiency with their abilities from other areas. The paladin mentioned above might not be able to survive as much damage as a knight, but they can heal other players and help them survive, something a knight could never do.
Historically, MMOs have had a great deal of difficulty designing hybrids that are powerful and valuable without completely displacing their parent classes. The catchphrase for these overly successful hybrids is “tank-mage”. This term comes from the early days of one of the first MMOs, Ultima Online, where some characters could both wear heavy armor and cast powerful damaging spells. A tank-mage could both take and deal a lot of damage, creating a character that was superior to any other type of character in most situations.
Since Ultima Online, other MMOs have tried to avoid this problem, but players inevitably gravitate towards the latest incarnation of the tank-mage whenever possible. This is not a sign that the players are cheating or deliberately trying to abuse the system, it’s just the natural result of players trying to find the golden path and “win” the game. A character who can take more damage is better and a character who can dish out more damage is better — therefore a character that can do both is ideal.