This item, like the elemental obis that came before it, doesn't just force day/weather bonus proc, it also forces the penalty for opposing element. With the old obis this wasn't relevant, but since the new obi covers all elements at once, this actually makes it a downgrade in instances of opposing elemental day + weather.
For example, let's say you want to nuke with aero during wind weather, but it happens to be iceday. With the old obi you would get the guaranteed +10% wind damage from the weather, and a low chance of a -10% wind damage proc from iceday. With the new obi you would get guaranteed +10% wind damage bonus and a guaranteed -10% wind damage penalty.
In that scenario the Hachirin-no-Obi is just a wasted equipment slot, but in other cases it could be much worse. For example casting a cure in dynamis (double dark weather) during lights day would guarantee a +10% bonus and a -20% penalty.
Shouldn't this item simply be saving us seven inventory slots without penalty, like the Fotia gorget and belt? Does it really need to be situationally worse than the old obis?