I can think of exactly zero instances where you'd ever need turbo functionality on buttons III-VI.
I was just thinking about this, and there is definitely a use for turbo switches on other buttons. With a default button layout in Street Fighter II', the I and II buttons that have turbo are roundhouse (hard) kick and forward (medium) kick. None of the punch buttons have turbo, so if you were playing characters like blanka or honda you wont be able to use turbo to activate electricity or the hundred hand slap.
Also since jab (light) punch and short (light) kick do not have turbo, characters like ryu, ken, or zangief cant use turbo to execute rapid shorts or jabs.