My main interest in FFXIV is doing quests. Every time a new update goes live I rush to do quests. I like the storyline, the atmosphere, and the actual battle encounters equally. Imagine my disappointment when I encounter a quest where a group of enemies pose absolute no threat to me. Because of this, I haven't even been leveling up my main job, because I don't want to easily overpower content like Ifrit. One of the reasons I stopped playing FFXI is because I didn't like the direction they went with mission battles after CoP(with removing caps from CoP being the last straw.)
So, I would like the option to level cap myself(and my party) so I can have as much fun in the battles as out. This change would go a long way to my enjoyment of the game. I understand they should be optional because not everyone wants challenging content.
Aside from basic implementation there are two things you could consider adding to make it more accessible and popular to us:
1) Minor Exp or company seal or etc award for completing a battle this way. (Rewards can get more creative, some like Ifrit's Lamp(I think that is was called) which increases rewards in repeated battles would catch more attention than a paycheck.)
2) Once the player has opted to add a level cap to the battle(probably either as a third option in the duty message or another window after) allow the party time to adjust their Actions and Traits and Equipment.
It probably wouldn't be a good idea to add this to Guildleves if you want players to advance through camps. However I don't see why it couldn't be added to quests and sidequests though.