In-Game Parser.

[Viridiana]

I saw as many tank that sucks at proper damage mitigation (some of them even forget completely about their defensive cooldowns) and healers that are very bad at conserving mana and those overheal% are abyssal as possible as crappy dps, and those numbers are not visible anywhere.

Except that they are. How do I know? Because you noticed them without using a parser.

Know how many times I've had healer friends complain about tanks not using CDs at all? Or how many times as a BRD I looked at the WHM and asked "Why do you need ballad? You haven't had to res anyone." So, yes, your mystical non-transparent examples are still perfectly visible.

[Kazumac]

Except that they are. How do I know? Because you noticed them without using a parser.

Know how many times I've had healer friends complain about tanks not using CDs at all? Or how many times as a BRD I looked at the WHM and asked "Why do you need ballad? You haven't had to res anyone." So, yes, your mystical non-transparent examples are still perfectly visible.

While you can get some feel for this without parsers things like an OOM white Mage are easier address with one. Was it over heal or bad spell choice? If it is improper spell choice what was it specifically and what was their spell usage breakdown? Ive seen it used extremely positively this way to help a person who just had no idea what they were doing wrong.