You’re flat wrong about the frequency of the Pope speaking ex cathedra.
There have been several heresies denounced via such pronouncements.
Papal succession is but one example of the Authority vested in Peter, via the promise that “Whatsoever you bind on earth will be bound in Heaven.”
Your clarity on a single passage is hardly evidence that you need no guidance on the vast remainder of Scripture.
There is no set list of ex cathedra teachings, but thats because there are only two, and both are about Mary: her Immaculate Conception (declared by Pope Pius IX in 1854 and grandfathered in after the First Vatican Councils declaration of papal infallibility in 1870) and her bodily Assumption into heaven (declared by Pope Pius XII in 1950). - See more at: http://www.uscatholic.org/church/2011/05/there-list-infallible-teachings#sthash.1XhxXjYk.dpuf http://www.uscatholic.org/church/2011/05/there-list-infallible-teachings
I'm done with this conversation.