Senko
Now you're just being pedantic, and you know it

And yes, Harlequin could cast a Force 30 spell and the Perception test to notice him casting it would be 1. That kind of spell would probably be felt quite strongly by magicians in the immediate vicinity, by the way, given that his signature would be imprinted for 30 hours or whatever his Metamagics reduce it to.
But to address your point; you're emphasizing three words out of a long paragraph; the fact that the writers felt the need to call out that magic is rarely subtle is kind of my point. You may not play it like that, and that's fine; your table, your rules. If you want magic to be subtle, do it. But the writers here are saying that magic is rarely subtle, and that it's sometimes A, sometimes B, and sometimes something else. Those uses of "sometimes" does not preclude the use of "rarely" as you seem to think it does, instead it emphasizes that point.
Again, the only time Magic in Shadowrun is subtle, at least according to the book, is when the spellcaster, conjurer, or what have you, is working with Magic that is of a Force significantly lower than his ability to handle (i.e. Magic Rating X - Force Rating Y = Perception Threshold of 1/2/3 or higher as dependent upon your table's use of the Perception skill amongst NPCs).
That being said, let's also keep in mind the following: most NPCs are about as observant as your average storm trooper. No Perception skill and an average Intuition of 3 means you're rolling 2 dice; that's an average 1 hit 55% of the time, but also an average critical glitch
27% of the time 2% of rolls, because of that sillyness where glitches now require more than half", and not just "half". And that's before you figure in modifiers like the -3 for not specifically looking for something, and/or the -2 for being distracted. So yeah, things are not quite as black and white as you seem to think the rules indicate. Again, the rules are a framework; use it to make the world come alive as you like. If the Perception test as written doesn't work for you, then change it. This is the general discussion forum, not the rules forum; RAW doesn't matter quite as much here as it does there if you ask me, but the rule is there if you want to use it or not.