But yeah, form V isn't exactly a very sound policy, unless you're charging hordes of stormtroopers. It's emphasis is on reflecting and redirecting of blaster shots. It's rather like a perverse version of Form III, lacking the actual tight-knit defensive moves
Against blasters form V is for reflecting bolts back at the person who shot them, form III is the one the just deflects them.
And form V is more than just that. Form V is an offensive style that uses aggression, and is how Anaking owned Dooku, it isn't nearly a useless form.
Obi wan's offense in soresu is worse than chewbacca's breath. besides rotj vader seems to like to use force choke, a move which if you saw what dooku did to ros obi wan, obi wont have a good defense against. and vader at this time is much too calm to try a 60 foot force jump like in rots. so vader wins 7/10 times. the extra three are incase obi wan can get close enough to vader without getting force strangled to death.