I love repeating myself...
Firstly, we can balance duels ourselves very easily. Want a serious match? Balance it yourself, same Job verses same Job, or similar. No shit Black Mage verses Monk wont offer much.
Secondly, one sided matches are a valid form of testing yourself. If I duel a DPS as a Healer would I be aiming to win? Not really, I'd be practicing almost the exact situation I'll be faced with in regular PvP. That would be the point. I tend not to heal in Frontlines because I know I suck at it. Were I to practice I'm sure I'd get better, but for the duration of that practice my team is at a disadvantage. Being able to lightly practice outside of any team based PvP would be a significant boon.
Thirdly, who cares if a fight is unbalanced? Were I ridiculously confident in my Black Magery I'd quite like the chance to try and prove it by burying a melee DPS.
Lastly, Duels would offer something to do during queue times (which is PvP, before someone pops in claiming I should be happy to queue for PvP and then go mining to pass the time... No, when I queue PvP I want to PvP, no content scratches that itch between queues except potentially Duels) and it wouldn't be significantly hard to implemented if balance goes out the window. Lock it behind Friend Requests or a PvP Rank requirement to prevent harassment, both are systems already in place. Only allow them in certain areas? Another system already in place, with Triple Triad Duels. All they'd need to do is temporarily unlock PvP skills (done in PvP and Wolves' Den Pier already) and work on an iLevel sync to the lowest player, which is something they already plan to implement as well.
There isn't really a lot of work that would need to go into implementing Duels and they offer so much; PvP to do while queuing, a way to practice against a living target, a way to learn various Job strategies, something to do for a serious challenge, etc. etc.. Heck I'd quite like the opportunity to just test things without having to drag six other people into Wolves' Den, for example how much Accuracy negates Blind.