Well there is the issue with encounter design being lop-sided, the issue regarding player skill increasing and also increases in ilvl as well. This is also not including solo encounters also being boring considering that you need to DPS in those as well.
It's simply not feasible for the developers to put in the time and effort to make all encounters balanced around 80% healing. Yoshi-P would rather spend his focus on making sure BLM is the best designed class in the game.