IIRC loot adds about a 1.5x to the roll and rewards are no different, not sure but some maps seem to take longer with MM :confused: By the default values, it starts at adding roughly 10% to the value of the wave number, but it's worth proportionally less when you start adding in factors like NM and HC. That line of code is essentially (wave number / scaler) * (Base Multiplier + MM Multiplier + NM Multiplier + HC Multiplier), so the more modes you have on, the less bonus each gives proportionally (a little diminishing returns).
At best it's about 10%. At worst (with default values), it can be worth as little as 5%. I think I calculated it as 2% for NM HC MM Palantir a long while back, using the values from DDDK's data for that map.
Whether MM is worth using is down to if you can consistently beat it, really. Sure, 2-5% isn't a very large bonus, but if you're beating the stage easily with MM on, you may as well take it.
Ed: I'm not really giving an explanation of what the %'s are meaning here... simply put, MM is worth 2-10% of the final multiplier used by the RNG to determine stat rolls (all stats except resistances and SPS/projectiles). So if your multiplier/final stat roll is 20 without it, it's 20.4~22.0 with MM on. Matters more for weapons and accessories than armor in the endgame, because armor multipliers cap low enough that pretty much all DLC maps can reach the multiplier cap on NM HC.