The problem is how we look at the ability, because it says cost 18 MP we 'assume' it costs just 18 MP. What is really going on behind the scene is at level A you have a base of B resource and ability C costs D percentage of your resource pool to cast.

What this means is in a fight you can cast Flash let's say 3 times. As you gain more mana you don't get to cast Flash more than you used to. The mechanics are based on learning your class, understanding the abilities, and using your limited resources to the maximum values to complete the objective (Kill the enemy). If this mechanic did not exist you could in theory cast an infinite amount of times if you had the resource pool to accommodate as such. This is also why we have diminishing returns as GLA can recover MP and in theory cast Flash forever but as you cast each flash you will notice it lasts less time and eventually starts to have 'no effect'.

This allows for better more challenging content and reduces what I would call the WoW problem of scaling and having to reinvent the game every expansion pack as they were unable to foresee that a 10% increase to X ability could scale to insane values over time or changes to stat allocation or level caps.