I fully support this. The number of times you see a tank or healer drop from levelling roulette when they get a dungeon WAY below their level defeats the entire point of the roulette. FYI whilst the bonus does scale as others above have indicated it does NOT scale enough to match the amount of XP you lose by doing a low level dungeon. Doing a level appropriate duty versus Tam Tara or such will still net you considerably more XP overall.

To be honest I wish they'd just kept the XP mechanics from FFXI. As it is now in XIV the amount of XP you need to level is exponential whilst the XP given by mobs is based on their specific level but cut off if you're too far above them. The result is that if you level sync you get XP rewards from low level mobs but the amounts they grant is still specific to their level which is stupidly low due to the exponential growth of the XP curve.

In FFXI the XP you got was simply dependent on the difference between your level and the mob's level. This meant that no matter what level you synced to, the XP you got was always the same due to it being based on relative levels, and not static amounts for a given level of mob. A level 1 player would get 500XP from killing a level 1 mob. A level 75 player would get 500XP from killing a level 75 mob. This was a much more player friendly system when you factored in level syncing. The XP curve would need to be adjusted to fit the change but that would be a simple fix (and one that many MMOs have undergone during their lifetimes so I'd say we're well overdue such a change).

I believe the reason they changed it was to stop players farming lower level areas for XP. They wanted players to do level appropriate content. I don't think that concern is really valid any more though. Since we cannot level sync outside of duties or fates there's really no risk of seeing the likes of Valkurm Dunes or Qufim farm parties returning.