Personally, I love how I can level every class/job on my single character. It means I don't have to pay for the same thing on multiple different characters (or fighting the same boss over and over for unique/untradeables) and it also means that Square-Enix have licence to make some fights more suitable to certain classes than others. I long for the day (although I imagine others dread it if they haven't leveled multiple jobs >_>) they make certain bosses resistant to certain types of attacks, meaning people will finally have to adopt extremely different class-type-strategies between enemies rather than stacking on BLM or whatever. If people were locked into classes there would be no way to do this. To balance things, they could make it so that each job has at least one major fight where they are useless, so as to not pick on any specific job.

Because of this system, they can try new things like the above.