Of course soloing at lv50 is a much easier task for your job, however doing it from 30-50 is far from a breeze when compared to your class. You can die a lot easier when your on a job instead of your class.
GLA/PLD: soloing as a PLD from 30-50 can mean life/death when you can't rely on your free instant heal second wind. Sure you can cure yourself but in most cases(like maxing guildleves) you need that instant heal to help you stay alive mixed with your cures.
LNC/DRG: DRG has the damage but again from 30-50 not having those safety nets can get you killed a lot more than just switching as your class.
No point in just repeating the samething so I'll stop here. DoM might be the only exception(not much of a mage guy) but for a DoW 30-50 can be slept through as a class. Jobs will require you to stay more on point unless you are soloing very easy mobs.
As stated it's not that jobs can't solo and classes can't party. It's just a shift in playstyle with different pros and cons. When it comes to soloing you should not be dying more than on your job and in a party your class shouldn't be out damaging it's respective job.
They each give an edge in certain situations but the edge is not too big to make up for it. A class will bring less damage to a party but brings in support that its respective job can't do. A strategy is a strategy and regardless if you use it or not, it doesn't make it any less viable if it executes and gets the job done.