I'd say that you're not lost, just overthinking the situation. Think of those weapon training prerequisites as being there for exactly that - you need further training in order to use that weapon. As a privateer part of your training is to be able to use a sword and shield. Similarly buccaneers are trained to use large two-handed. If you want to use heavy, destructive, and otherwise unwieldy weapons like them (or staves that channel powerful magic), then you need further training to use them properly.
Gameplay-wise you could view it as a way for each class to have its own signature weapons and combat styles, and in order for other classes to use them, they need to spend the practice points to get those signature weapons.
Canny Matthew Tolliver Able Caleb Tolliver Clever Alexis Tolliver