Ok, I think I can answer this question.
I started out as a whm, took it to 61 and started to fall asleep during parties.
I started to lvl ranger, its 72 and climbing fast.
So I think i have good incite into this. But let me say this first, if you have not played a whm, rdm or bard, you really have no right to complain. What I have notices is melee and blm jobs are extremely easy to play. They are mindless jobs more or less. Sorry if this offends. I can be half asleep in a party and still be a good ranger.
Playing whm to about 60 is fairly easy, cure, haste, regen, rest. Not much to it. Too things to not, haste is nice but people do forget that it costs mp, if your whm is good you need to trust them to use mp wisely. Sometimes that mean noone gets haste accept nin tank if it would hinder chains. Haste really doesnt make a huge difference as many think. Chaining is the number one priority and mp is needed to do that, sometimes you can haste sometimes you cant.
Ok now for my explaination. At lvl 61+ things change in battle. Mobs you start to fight have status effect attacks. This can cause things to get a little crazy, i know i started lvling whm. Main healer at this lvl becomes the hardest job in the game hands down. You are watching health, trying to watch log for status effects are learning the sounds of the attacks that cause the effects. Tanks should have macros to let mages know when they are paralyzed, "stoned" etc. It makes things easier to see cause the battle text can go buy so fast. Be patient.
So now you are curing, hasting, erasing effects, regen, medding, barsomething, it is very tiring. Where as a melee you can do your job and sit back and relax until you tp is ready. Mages will know what I am talking about.
You might be thinking well before crabs have str down aoe etc, that dont count, they can spam that 3 times a fight and most times its just not worth it to keep castign erase cause by the time your done erasing everyone the crab will aoe again, so most whm will ignore it more or less unless the fights are easy. Too much mp used.
So this is why at 61+ whms can seem not as good.
Plus there is an atitude towards whms and melee think its an easy job. Again melee is a brainless job, i know i am one too. I will give an example. My first pt back as whm i lvled with friend in cape. We had no refresher. So I had to change strategies. Noone got hasted exceptthe nin tank. I regen, cure med, and repeat. I probably medded about 80 percent of the fight to keep my mp high. Now a good whm will know how much damage a mob is capable of doing. Why is this important. As mages no, the more you med the bigger chunk of mp is restored every tick. I let the tank get into brown so I can get as much mp back as possibe. Was the tank in danger of dying, no, because i know what damage the mob is capable of. So without a refresher I was able to keep mp high and we had absolutly no downtime and we chained 5 every fight.
So I asked a couple members of the party that I know how i did cause it was my first time in about 8 months. The two of them said, so so and ok. I asked why, they said you let the tank get into brown and he almost died a couple of times, then you used cure 5 and thats too much hate.
Well if you a whm you know that cure 5 has maybe the same hate as cure 3 and is the most mp effiecient cure spell. And as I stated the tank was in no danger of getting killed cause i knew the damage the mob was capable of. They also said didnt get hasted, again this goes back to mp eff.
Now I ask you, a main healer, with no refresher able to keep no downtime and chain five consistantly. Is that a good whm or a so so whm.
This is the ignorance that healers have to deal with. If hes good and you trust him, trust he knows what he is doing. If you have never played a main healer at a higher lvl you really cant give an opinion on the argument.
Ok I am done, that was long, I am sorry.