No skill needs to be anything, unless u want them to
The reason why people choose to have bushido at 120 can be any or more of the following.
- Less steps to archive perfection.
- Better chance to make critical hits.
- Better chance to control lesser hiury
- More damage with Wirvelwind weapons.
- Confidence gives nice healing, (even on archer templates).
The reason why people choose to have 120 weapon skill + 120 bushido + 60 parry is to archive max mana reduction for special moves.
They could in theory choose 60 poisoning over parry, but it dosent scale so well with bushido.
For further reading / understanding, they nice people over at the warrior forum, made a very cool guide u should read.