If you want different weapon sub-categories to be mechanically different from each other, then yes, sub-categories are obviously needed.
Exactly.
The topic here is to have different types of swords etc.
I mean, pretty sure Blizzard already confirmed that there will be different types of swords. Based on the region they drop etc. So the only thing missing is the different characteristics for those different swords.
To make weapons more interesting and diverse. To make it matter if I am using a sabre, or a long sword. Etc.
If they had different sub-categories with different characteristics, it specifically would no longer be based on random rolls. A Scythe would always roll with the characteristics of a Scythe. A Bardiche would always roll with the characteristics of a Bardiche.
Unlike if you just make it all randomly roll like if it is Rare items.