Stance changes being able to have their own Play On Start animations to allow easier transitioning for different positions without unintended interference (limb twisting, self-stabs, etc.).
It'd also allow folks to make stance changes that begin with a move (throwing a weapon from your off-hand, such as a dagger or shortsword, to dual-grip a larger weapon in the main hand) or stylistic beginning (a flourishing spin before your sword is held above you, for instance).