I'd love a change grip button, and for more than just spears going from short form to long form (which btw is the technical term for everything you guys have been debating, and both are valid based on situation. Though I agree with Icarus in that this game is NOMINALLY at least about group tactics and not 1-on-1 dueling, and the formations in general need to be improved cough spear wall cough)
A change grip button would allow us to do so much more than that:
Swap bastard swords and other either-or weapons from 1-handedmode to 2-handedeven if a shield is on your back
Use throwing weapons as melee weapons
Use two-handed swords in half-sword grip - or better yet mortschlag grip
Use a shield as a primary melee weapon
Desperation melee attacks with a crossbow or bow
The possibilities are endless