The way I see it, there are two extremes: either define a primary attack mode with other modes as options, or use the best rules out of all available modes. There's a range of possible rulings between those extremes, as you can see in these possible rulings for the broadsword example:
- Pure Primary + Option: A broadsword is a hacking weapon (1d6 damage + bleeding if not wearing hard armor.) Player can opt to slash, to avoid accidentally severing body parts. Player can opt to stab (best of 2d6,) but risks getting the sword stuck.
- Upgrade Option: 1d6 damage + bleeding, but if the damage rolled is low, the player can opt to declare a stab attack after the fact and roll the extra d6.
- Downgrade Option: Best of 2d6 + bleeding, but player can opt to declare a hack attack after the fact to avoid a stuck sword.
- Best of Merged Modes: Best of 2d6 + bleeding, no chance of stuck sword.
I think I prefer the first approach, especially since it fits with weapons with special attacks, like the flail with its option to disarm. What does everyone else think?