In recent years, and I would say starting with the Star Wars prequels, it has become a trend to tell origin stories. Star Trek itself hasn't been spared with the recent reboot of the franchise. I think that ruins very much. Do I need to know why McCoy is being called "Bones" by Kirk? Do I need to see how Kirk cheated the Kobayashi test? And with a quick back nod to Star Wars, having just re-watched Empire Strikes Back, I noticed that so many things that were told in detail in the prequels should not have been told at all from a narrative perspective. Sometimes, things are better left implied.
With Guinan, the very appeal of the character is her mystery. She occasionally hints at things of her pasts, or slips a mention, and it's left to the viewer to piece it together and find out for himself how she can know all the things she knows. Generations added a bit to the back story, but in a way that it actually raised more questions. If Guinan had been part of a narrative of, say, the past ten years, we would probably have gotten to see everything: Where she came from, what exactly her powers are, what kind of experiences and adventures she'd had... the aura of mystery that surrounds her makes her such a memorable character, and I'm very happy that this has not been tampered with. Yet.