Sometimes, whenever I see fantasy readers laud a work for the rigor of its magic system — we’ll come back to this word “system” later — I wonder: why are these people reading fantasy? I mean, if they’re going to judge magic by its similarity to science, why not just go ahead and read science fiction?To a certain degree, I think Jemisin has a strong point - fantasy isn't necessarily about clearly organized and logical systems. Magic is often something inexplicable and, well, magical. Not everything has to make perfect scientific sense - indeed, many of the cornerstones of fantasy literature forgo such systematic magic systems. The best fantasy succeeds in creating something so other that it just doesn't need any kind of system or justification.
But I disagree with Jemisin to dismiss magic systems overall. First of all, I think there are enough readers whose tastes overlap enough between science fiction and fantasy to justify having the lines blur a little. Many of these magic systems Jemisin dislikes are, in fact, stand-ins for science in those particular worlds. It's not science, and it's not science fiction, but it's fantasy that does appeal to sci-fi lovers... That's not a bad thing.
Then there's Jemisin's complaint against magic systems as a whole. Here I must politely withdraw - Jemisin sees a much bigger, looming issue in the concept of a "system" than I ever will. I know what underlying problems she's referring to and recognize them, but I really don't think that they have anything to do with the fact that certain fantasy worlds have particularly organized and detailed ways to conduct magic. Correlation does not prove causation.
Lastly, I must admit: I kind of enjoy magic systems. As I mentioned earlier, many writers who use magic systems in their books use it as a form of science. On the other hand, the vast majority of fantasy that does not take place in modern-day Earth (and sometimes even those stories that do) completely eschews the mere concept of science. It just doesn't exist. As a scientifically minded person, this can sometimes come off simply as sloppy writing. A well-structured magic system can, however, make a fantasy world seem a lot more believable - it shows that the author put a lot of the world-building. It's obviously not the only factor involved in my appreciation of the book, but like originality, it can add a whole new dimension.
The source behind Jemisin's frustration? This:
I’ve seen these folks, most of whom are future fantasy novel-writers, positively agonize over their magic systems, taking great care to consider rules, required resources, the laws of conservation of magic, yatta yatta yatta, all for fear that they’ll get published someday and have their magic systems picked apart by the Fantasy Police. In some cases these writers had spent far, far more energy on trying to create a magic system than they had on trying to create plot or characters. Sadly, I’ve seen this same kind of to-the-exclusion-of-all-else focus on mechanics in the works of some published writers — and worse, I’ve seen readers going ga-ga over this sort of thing, as if the magic system really is the only part of the story that matters.Here I once again completely agree: magic doesn't have to be organized. In fact, oftentimes "organized" magic systems aren't pulled off well and, like Jemisin points out, it ruins an otherwise interesting story. Writers shouldn't have to bend over backwards to explain how their magic works, they need to make sure it fits their world comfortably. For some worlds, this will mean that magic is like science - it's logical and clearly explained. For many other worlds, however, magic will simply be magical. Both are fantasy, and both are fine.