Let's SE determine what is and isn't a waste of their time.
The game isn't only about "functionality". A NIN isn't a ROG and a ROG isn't a NIN.
If they do extend classes into full Jobs, what makes you think they won't add Job specific abilities to "differentiate" the 2 jobs?
Also, again, so what if they are similar mechanics-wise?
If you really want to "not waste time", might as well just have "fighter", "thief", "priest" and "wizard", then call it a day.