Totally a tangent, but he's right about that. It was a flaw in Harry Potter as well. There was no logical system to how magic worked; spells did whatever plot requirements said they did. And it detracts from the sense of realism in a world when the magic just does whatever is needed at the moment.
I take significantly bigger issue with the lack of societal change from having magic. Way too much of wizard society was “Muggles + occasional party tricks”. When you can conjure food, water, automatons, etc from nothing, nature of living would change completely.
You can brew luck? I would be mainlining that stuff every day. Time travel is given to children? Why is there a train when there are a dozen different ways of magicking yourself around the world?
Harry Potter and the Methods of Rationality touched on these inconsistencies.
Compare to, say, "A Wizard of Earthsea", where magic is explained in a different way that points out that while a wizard could transmute one substance into another, no wizard would, because of the far-reaching ramifications.
The system was not fully elucidated by any means, but the subtlety of it was suggested by such things as Ged deducing that the doorkeeper was one of the seven masters of Roke.
Well, magic still needs to follow some kind of rules for it to be usable. Otherwise "magic" would just be something random (or maybe chaotic - we just haven't figured out the rules well enough).
