I can see where you're coming from here.
I've never encountered a situation as bad as the one you described (a whm playing the game for a week and not even knowing how ESUNA works), but I have seen other detrimental effects of power-leveling. Sometimes, when I'm in a group, someone will ask, "Does anyone have [job x]?" There is always someone that says, "Yeah, but I don't really know how to play it." That's pretty frustrating.
I like learning a job's abilities as I go. It's exciting every few levels when you earn something new.
Yes, PLing makes for faster exp, but if you don't plan on playing the job, and don't know how to, what is the point of leveling it in the first place? ...So you can have your crown? In the end, being powerleveled to 50, in my opinion, removes the joy of leveling in the first place. It also makes it somewhat harder to learn the job if you don't receive abilities 'as you go'.
That said, it's a personal opinion. If powerleveling is possible, people are going to do it, and I'm not going to knock them for it... BUT, if I'm making a group for something, and you join as a certain job, I expect you to know that job as well as a person can. Some players are better than others, but 99% of WHM's should know what Esuna is and what it does. When your lack of knowledge affects an entire group's ability to succeed, shame on you.
Otherwise, your choice to powerlevel yourself will ONLY affect you, and in that case, I don't really care. You're only hurting yourself.
