Well, several things.

The ability to level more than one class on a character encourages people to revisit those dungeons the second, third, fourth, time around since there will not be enough quests to level on.

Level sync in dungeons.

Yoshi-P has said that for now it must be a party of four players but once people move on and become more endgame focused, he will relax the entry requirements allowing you and a friend to do a dungeon with a Chocobo Companion each or even solo them.

Also this game caters to people who are newer to the genre so not every player will get all level 50's within a month.

I do see your point but it has and will continue to be addressed.