So... are YOU going to zero your rifle at 100 yards if you anticipate a 400+ yard shot??? Sure, you can learn to dial in elevation from any point, but there is not always time to dial up a turret in real world hunting situations. Even BDC reticles have significant variables and most start the algorithm with a 200 yard zero and then adjust from there. Do as you please, but if shots are likely to be 300-400 yards I will never recommend zeroing at 100.
The Zero will get put at wherever it needs to be depending on the situation. That's the beauty of it. But yes, it always starts at 100 yards.
Time is a variable, that can't often be fully controlled. That's part of hunting. Distance can, and usually does buy that time. If there isn't time, then there isn't. Rather let it go, than guess at a holdover, take a shot, and hit a jackpot.
If the BDC reticle is to be used, and it does get used, that option is only 6 to 10 clicks away, depending. Hell... we even use the same scopes, and the same reticles. These wear M1's with a zero stop. Tough act to beat, for versatility.
Haven't been on a long range course, or met a long range instructor yet that doesn't recommend a 100 yard zero as a starting reference point. It is simply the easiest and most common starting line. That's just a nod to their experience and preference, over yours...
Also notice that it isn't being said that it's the "best" or the "only" way.
It isn't for everyone, and that's fine too. Or, at least it should be, right?
R.