It's a multivariable equation. MBD2 is a straight multiplier, but it's also diluted by other MBD. If you're sitting at 60 MBD (emp body+3, ea hat +1, max agwu hands, mpaca's staff, capped MBD1), then mujin band is worth 1.65/1.6 = 3.125% damage.
At 400-500 dINT on Thundaja, value is 1 base per INT. So, the 16 INT from a metamorph ring +1 is worth 16 * MAB (~5.2) * 1.25+(MB) * 1.6(MBD) * 1.25(storm) = 208 damage. So, the Mujin Band will almost always give more base damage on very weak targets, but giving a concrete answer requires your full stats and your target's INT.
If you reduce the dINT to 0-49, the value per point changes to 4, so you get 832 damage in the same situation. In this case, the metamorph ring+1 is more competitive[but likely still loses]. It drops to 3.75 at 50 dINT, and stays there until 200 dINT.
In practice, the extra m.acc provided by 16 INT and 15 m.acc is needed on high end targets and with the greater value per point of dINT it's an easy tradeoff to make. If you're nuking something that won't resist you, you don't need it.
Edited that a couple times. Was thinking Mujin was 3%, but it doesn't really change the takeaway.