We literally have this discussion at least twice a year.
You know what isn't fun? Having to press five times as many buttons as the other person with the same job as you to do the same amount of damage because you syncked down to their level.
Also not fun, dungeons taking longer because people who are syncked down don't know how to play their jobs at their real levels, so they're only doing one fifth of the damage they should be doing because they're not doing their full real level rotations correctly.
There are too many sync issues to deal with when balancing people with no sync vs people with max level sync. It's far safer to just give everyone the same boring restricted button set at syncked levels.