The point of having at least a job at 60 in all three roles is that you can actually say "come to Gridania/Uldah/Limsa, I'll show you a few skills from each roles and how to use them on dummies".
There'll be quite a lot of people in the mentor chat, so meeting newbies will be a key point to show stuff to them. If you only have one job at 60, all the "teaching" you can provide will only come from text... and at this point, telling them to read a guide would have mostly the same effect.
Also, don't forget that the mentor system is also something that was made to help newbies get hooked to the game and start subscribing. Meeting veteran players, joining FCs and getting a good glimpse at what a well all rounded character looks like is a very important point.
A dude with only one job at 60 is kind of boring.