The rules for any magical system are modelling a fantasy, and therefore can be devised however you like. The rules for modelling real-world physical systems should model real-world physical systems at least to some playable degree. Limitations on how many times you can jump, or swing a sword, are arbitrary in ways that limitations on magic (which brings no real world expectations) are not.
That paragraph deserves to be in the FRPG Designer's Bible.
I completely agree.
But I would like to embellish that even magical systems can suffer from arbitrariness, in-game and meta-game.
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Welcome to the Dr Phyl Show! Today, we have a very special guest. He is a "living spell". Ladies and gents, please put your hands together for 'Beguiling Strands'.
Dr Phyl: So Beguiling Strands, you describe yourself as "strands of subtle magic that weave through your foes like a spider’s web".
BS: Yes, Dr Phyl, that's absolutely right. With all my scintillating colors and gleaming lights, I can really confuse people, you know? I can hurt them mentally, and they move away from me.
Dr Phyl: I see. Yet your first name is 'Beguiling', right? The definition of beguile is to deceive or charm. Yet you hurt and push people away from you. Don't you think your parents misnamed you?
BS: Now look here, I won't stand here idly while you criticize my parents. So maybe they misnamed me, so what?
Dr Phyl: Well, is that fair to you? When you introduce yourself, you're already giving the wrong impression. One might call them 'careless' in the way they named you.
BS: Careful, Phyl. I might just push you away.
Dr Phyl: Yes, let's discuss that-- your nature to always push people away from you. Now I've had flashlights shined in my face before. I've seen incoming car headlights. I might step back. I may cover my eyes and be dazzled for a moment. I might cower. I might run off the left or the right. Why is that you only and always feel the need to move people away from you? Why don't they react in other ways to your confusing lights?
BS: Are you calling me a one-trick pony!?
Dr Phyl: I don't know, ARE you a one-trick pony? I'm trying to visualize this, and I just don't understand it. Realistically, people have different reactions to the same stimuli. Yet you're the exception... have you met other magic lights before? Are they also so single-minded in their effects?
BS: I don't know. I never met any other lights, mundane or magical.
Dr Phyl: I see. So you're parents named you 'Beguiling' even though you're really not. They only taught you to do one single thing, over and over. And they never really compared you to other lights to see if you were reaching your natural potential. Is that a fair assessment?
BS: Hmm...
Dr Phyl: Is it possible to meet your parents and ask them why did this to you, why they put in this straightjacket, so to speak?
BS: No, you can't meet my parents. They are unreachable, and they are protected by a loyal legion of fans. I've never heard them acknowledge that kind of criticism.
Dr Phyl: That is unfortunate. I would like to free you from this self-constrained limitation. You have so much more in you to give to the world! You could move people up and down and left and right, if you just let yourself!
BS: NO! no! NO! All I can do is hurt and push. Hurt and push! That's all I want to do. That's all I will ever doooooooo....!!!!! [runs away, crying blue droplets of light]
Dr Phyl: That's it for today. Next time, I will interview another living spell, 'Hypnotism'. Hypnotism can seize momentary control of peoples' mind. You'd think that Hypnotism would revel in his almost unlimited potential. And yet, when Hypnotism invades someone's mind, he likes to do only one of exactly two things: attack somebody, or move. Why isn't Hypnotism more creative than that? Didn't his parents teach him to explore his full potential as an hypnotic power, or did they raise him to do only two arbitrary tasks? Is he misnamed? Is he a robot? I will try to find out!