>>1989200It's more complicated than that.
The top of your fork steerer is threaded and keyed for the headset. This section is weaker. The minimium insertion line is a guess intended to place the wedge below the threaded section of your fork. Actual good practice is to service the headset and check this, not just follow the line.
Unfortunately many forks are threaded all the way down for every size of frame, and then cut, so the larger size frames can have like 10cm+ of threads.
Installing a normal stem 'safely' on many forks is impossible.
That's on a lot of nice bikes too. So does it really matter? Lots of bikes have dangerous stems with the wedge on the threads, even if it doesn't look like it.
The failure is real but it's not that common. Also even if the fork splits, the wedge will hold both sides together. Your steering won't completely fall apart. There's a failsafe.
The more important thing, often neglected, is to thoroughly clean and have a decent gob of grease around the wedge and bolt on install and not go absolutely ham tightening it. It's a good thing if your stem would rotate in a crash.
Also important is to just inspect the steerer of your fork occasionally, on every bike.
tldr:
>Do I really have to abide by this rule?no
