Defensive Duelist requires wielding a finesse weapon, yet one of Monks' abilities is to treat Monk weapons (including staves and unarmed strikes) as "having finesse" (in all but name: it allows monks to use Dex for attack and damage with all).
Can a monk use any monk weapon with DD? If the monk is skilled enough to wield a staff/unarmed as if it were a weapon with finesse, it seems flavour-busting to have to wield a dagger or what have you, just to be able to trigger DD… "I wield a dagger… which I only use 1/round, sometimes."
If not, would a house-rule allowing this be overly unbalanced (i.e., allow exploits or some possibly game-breaking scenario)?
Best Answer
No.
The key here is that the Monk weapon ability doesn't actually add the keyword "finesse" to the weapon, it just lets you use Dex for attack and damage.
Defensive Duelist specifies that a weapon have that keyword, and thus, no, it can't be used.
Adding more fuel to this fire is Jeremy Crawford talking about natural weapons and coming down on the side that just because you are attacking with Dex does not mean you are adding finesse:
Would this be a game breaking houserule? No, that's quite unlikely. The Staff does d6 or d8 damage, and there is already a finesse weapon that does d8 (Rapier). The monk isn't proficient in that, so it's not available, but it's not out of bounds for them to be able to use it.
Note: Brian points out that two monk weapons are finesse weapons, and as such would be usable with this combination (and get the Monk's proper monk weapon die). These are the dagger and the short sword.