I think it is a good idea to left as it is, it should give players chance or reminders how to learn/do their jobs effectively.
Yes. I actually enjoy the level cap and restrictions in lower level dungeons, it gives me the chance to go back to basics with jobs and reminds me of a much "simpler" time. Admittedly it's a bit annoying levelling up and not being able to use your new skills all the time, and it's annoying when you have to sync to 49 for certain FATE's, but beyond that I'm not bothered by it.

The truth is, you don't need all your skills for those dungeons, they're not designed with that in mind. The addition of all the level 50 skills would likely make them even more trivial than they are already.