Diminished? More like quite substantially increased.

Maps.
Anything on a weekly cap.
Roulette bonuses.
The lack of craptastically rare drops that would make you throw your keyboard getting them on the wrong character.

The only comparable thing in xi was dynamis but I never had the time to do even 2 runs a day when I was running 3 characters there. Well, crafting you needed alts for there I guess. But certainly it's a no-brainer here to run 2 characters if you play enough.