Kinematics
Hero
For Fighter with a 1 level dip in Monk, it's a great boost for up to Fighter level 4 (character levels 2-5), but collapses after that. If you're going for two-weapon fighting, picking up Dual Wielder gets you the same benefit as the Monk dip, and it ends up being a wash, or better for the single-class Fighter after that. (If you don't get the DW feat, the Monk dip is strongly ahead.) And if you go to heavy weapons, you can't use them and the Martial Arts offhand attack at the same time. Putting aside juggling shenanigans, a greatsword build with GWM outclasses the dual wield build.
Still, there are cases where I could see doing it. In a recent D&D game, it took us a year of play to get to level 4. That's a huge amount of time where I could have boosted my damage by like 50% with a dip. But if the game moves a lot faster, you'll be hitting levels where it's more a hindrance than a help. And losing the bonus Monk damage because you got a sparkly magic longsword which doesn't let you use Martial Arts would be kinda painful.
Numbers crunched in my personal DPR spreadsheet, which I've been rebuilding with new PHB info.
Still, there are cases where I could see doing it. In a recent D&D game, it took us a year of play to get to level 4. That's a huge amount of time where I could have boosted my damage by like 50% with a dip. But if the game moves a lot faster, you'll be hitting levels where it's more a hindrance than a help. And losing the bonus Monk damage because you got a sparkly magic longsword which doesn't let you use Martial Arts would be kinda painful.
Numbers crunched in my personal DPR spreadsheet, which I've been rebuilding with new PHB info.