Let's ignore the devs stance on this topic for a moment and let's ask the real question:
Why?
The only benefit from it is that you level two jobs at once. Aside from that, look at SMN and SCH. They share only one spell. So, what is the point not having two completely separate job instead?
Even their weapons are not shared (even though they share similar design).
So, adding a second job to each class is a bit pointless.