Part of the problem I think is, leveling the opening class through MSQ is very fast and fluid. You get a substantial amount of experience through first time bonus and one time questing. That in itself makes off class leveling seem slower by itself. It's why I switched gears and started leveling alts. Not only will I have 8 characters with unique classes and Jobs, I will have 8 characters, not being gated by timers and MSQ gating. For people with standard 3, you can have a Tank, Healer and DPS in a similar situation. Sure, you 'can' level all classes on one character. You 'can' do a lot of things in FF14. I wouldn't recommend a lot of them. You 'can' mine 18,000 nodes and get an out dated level 50 Luminary gathering tool. I'm not too sure why you would though.