It's Magic Points. It's always been Magic Points, and always will be Magic Points.
I have to say that people calling it mana has always been a pet peeve of mine too. There are also no such thing as Elves in FFXI, they are Elvaan and are pretty much the polar opposite of how elves are portrayed in other games/books/movies besides the obvious pointy ears. You don't hear people calling tarutaru hobbits because they aren't. I can't think of anything where elves were gorilla armed brutes with high strength, bottom of the barrel agility, dexterity, and intelligence. Oh and elves aren't ugly.
As far as the answer to the OP's question of why there are so many mages that act like that, it's pretty simple. Because people let them do it. There's a double standard with mages because they are "needed" but in reality what you need is competent ones not ones that don't do their job and just leech a slot in your party.
Even the gimpiest of melee can actually contribute something to the party so he's not a total waste of the slot by simply engaging the mobs. His damage and accuracy may be craptastic but it's alot different if say he only engaged two out of ten mobs you were fighting. Any reasonable leader would boot someone for sure that did that but they let crappy mages get away with doing virtually the same thing all the time by not doing what they got invited to do. If people refused to party with them then it wouldn't be an issue but that's not going to happen, ever. There are too many, "sum xp is bettar thn no xp" types who will gladly invite them just so they can fill slots and even more that will crawl along at 3k/hour for three hours and not say a word about it til the party is over when they then decide to bitch about them to their ls or come here to bitch. At that point you're just wasting even more of your time than you already wasted by staying in the same party with them when you could have been getting better xp solo or with a damned npc.