Honestly, this angle is overrated.
Healing people through their mistakes honestly doesn't make that big a difference in mainstream endgame content. And by the time it really does in Extremes and Savage, said mistakes are every bit as likely going to get them insta killed irrespective of how much healing you throw at them.
A truly bad healer is more likely to be dropping the ball and failing to cast anything rather than overhealing to the point of running out of MP, but either way you'll usually find mechanics getting them long before they run out of resources at this point in the expansion. You can verify this by looking at 24 man logs and looking at the MP resource trace of the underperforming healers. Even sifting through some absolute stinkers that I've endured, I'm struggling to find anyone that genuinely ran out of MP without themselves dieing despite the carnage going on around them.
Where it does start to really affect you as a healer is when you have to start hard casting raises. But until that point gets reached, eh, honestly makes less of a difference than many would expect.