There's only one thing we will hopefully learn with the help of this guide, and that thing is this:
Priests are not buff bots. Behind every Priest in the game there's a real person! Wow! And yet, most of the time they are treated exactly like buff bots. Bots, who are supposed to respond to command "aop" (aliases: "aop plz", "aop!", "aop aop aop", "buff!", "aura") by casting Aura of Piece upon you.
It's not only rude. It's abuse. Running to priests and commanding them "aop" is abusing them in very open form. See, every class has got it's own set of abilities and it's own way of playing. It's own way of getting xp and loot. And Healer's (most common Priest build) way of doing it is being in the party and healing & buffing party members. As Fighters way is killing things while being healed. Yes, nobody is obliged to party with anybody, if you feel strong, you can go kill things on your own. But don't ask for AoP then. Priests never run around commanding fighters to kill things and leave them all the loot (and XP if that was possible). As Fighters main advantage is ability to kill things, Priests is to heal things. If you deny a priest an oportunity to benefit from it but want to benefit from it yourself - it's downright rude and it's abuse, I don't care what you think.
That said, don't forget manners and common sense, as always. I often give away AoP for random people I encounter on my way, especially if I see them struggling with monsters. Without even being asked for. I always give AoP if asked nicely and, the most important thing, if I don't need the party at the momment. And then there's PvP, in borderlands I give AoP to everybody I see. When we need to win, there's no place for selfishness, even if I'm not in a party and the party doesn't accept me (there can be many perfectly valid reasons why they don't want me even if they have room).
But never give away AoP if you need party. Never give away it if you are commanded like a bot. Never do that if you see other lonely priests around - suggest inviting them and having AoP that way. Or, charge for it. I've never done that before, but I'm going to do. If you are bot it doesn't mean you must be a free one
"Aop!" - "Insert 100 k please".
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I was also about to write how to ask for AoP nicely, what are perfectly valid situations when you really could ask for AoP but you can't have Priest with you, et cetera. However, it's all based solely on common sense and good manners - and our guildies certainly don't lack either.