Read some writers guidelines out there and you will see what I mean. Established systems of magic are crucial.
The thing is that good magic doesn't need to abide by rules. A good writer creates their own limitations for magic to create conflict and maintain tension. I think this is what the editors mean by a "system of magic". They don't want a lot of stories with magic rescuing the characters at the last minute, with magic whisking characters away or solving problems that arrise, when they could have just used that magic in the first place to end the story. It makes for bad storytelling.
Adding consistency and predictability to magic just means a unicorn is a horse with a horn. An elf is just a person with pointy ears. A dragon is a big lizard. Etc.
I like it better when the dead king comes to life and his moldering bones attack the hero. Or when the creepy reflection comes through the mirror to choke someone. These instances defy physical reason. They are what play at the corners of our minds. Our irrational fears that we still hold even though science tells us they are impossible.
Fantasy resides in that place that still believes they are possible even though they shouldn't be. Those quick moments where we do a double-take and say "this isn't happening."
That's the world Joel Fleishmann lived in (from Northern Exposure). In a way I envy Joel, because these experiences still gave him pause. The rest of the characters had become jaded. Their system of magic was predictable. It was more of a mundane thing, whereas Joel had to find his science challenged every week.
I like my sense of wonder to be ignited in such ways, and don't think that consistency or applying rules to a system of wonder is important unless it negatively affects telling the story. We buy the ticket to the roller coaster to be frightened. Not to have a good place to rest from standing in line for two minutes. Same should be true of magic.
When you understand the limits of magic, it isn't as interesting. It just becomes high school chemistry. Same as monsters. When you realize Dracula can be killed with sunlight, wooden stakes, etc. you don't understand why all those eastern European villagers lived in fear of him.
As a reader, I want to be kept guessing. And if I learn the reasons, I want to be amazed by how little I still know.