Many developers look at the Holy Triangle the same way Sasha Braus looks at a potato. They can't imagine life without it and will make terrible decisions to get at it. This isn't entirely a bad thing as its an easy way to make combat more interesting, but thoughtless application of it will quickly result in an 'I've been here before' feel.

The draws of the Holy Triangle include encouraging interaction between characters to create meaningful combat and providing obvious roles for those characters to slot into. This makes it very easy for a player to pick up the game and know exactly what to do and also pushes a certain chunk of strategies into the forefront, making it much easier to organize things like Raids in an MMO.

Allow me to explicitly explain the parts of the Triangle.

There is a Tank who has some way to draw enemy aggression and some way to soak up damage directed at them. Usually, this happens through taunts which make enemies attack the tank and high defensive stats, though occasionally tanks will use evasion instead, such as Warriors in Tera. While this leads to a few, obvious gameplay decisions, it can very quickly devolve into a 'how well can you manage aggro?' game. In an MMO, this isn't so bad, as the Tank player is probably very interested in aggro juggling. In an RPG, or any single-player venture, it severely limits the player's strategy by forcing them to adapt the Holy Triangle into all of their plans. Either they do things the developer's way and manage aggro or their strategy fails. Tanking is more interesting when, like other strategies, it involves making some trade-offs.

Next, there is someone filling the Spank role. This one isn't as game-warping as a Tank in most cases as the party isn't so desperately dependent on them. Provided there are other characters that can do damage, the Spanker becomes more of the icing on the cake. It becomes an issue when there is no viable way to get damage done without them, in which case there is a large deal of annoying babysitting in order. This character should be pure reward. Therefore, it's not a good idea for every character to be a Spanker, lest combat devolve into a who-can-blow-who's-face-off-first. Strategy dies a terrible death in this case as the only option is to go all out on offense as quickly as possible, similar to a DPS Race.

Finally, we have the Heal(er), sometimes referred to as 'Wank.' Their job is to keep everyone alive, though in a full Triangle application, this boils down to polishing the Tank's armor every now and then. They usually have various buffs to give them something to do in the long periods between when the tank actually requires healing. Often, they stand around doing a whack-a-mole healing job and tossing out buffs in the meantime. This is about as interesting as it sounds and severely limits the decisions the player is able to make.

Now, let me say again that, when properly tuned, these elements are very useful in a game. But when they become the focus of the combat rather than design consideration, the player finds that the bulk of their decisions have been made for them already. In some cases, this works, like in MMO Raids. In others, it's dangerously boring.

Next, I'll talk about Enemy Design and how it affects combat.