There's an existing 179-page thread about this here, which addresses the issue.
Some of the common counter-arguments to your concerns would be:
- If/when these are added, they will increase your level to a maximum of ten levels under the existing cap. Players would have those remaining ten levels in order to learn their job, and the later levels are far more reflective of how the job actually plays than lower levels, when you're still missing the majority of your job's abilities.
- Players can currently level entirely through FATEs, which can very easily allow them to get all the way to the cap (rather than 10 levels below it) without ever learning correct rotations, how to play, etc.
- Players who care about their performance are going to do the necessary research to learn how to play their jobs at the level cap regardless of how they leveled. Conversely, players who do not are not going to do the necessary research, regardless of how they leveled.
- Players can currently jump into Palace of the Dead an immediately play a job at 60. Should this also be prevented, on the basis that they may not know how to play the job correctly? Conversely, can this be taken as evidence that players need not have leveled a job all the way to the cap (regardless of method) to be capable of playing it at an acceptable level?