I've always been surprised that the Boon or Bane mechanic never made it into the Mongoose combat rules.
For two weapon fighting for example, both attacks could be made with a bane rather than the -2 DM.
With DMs the task can potentially become impossible, or impossible to fail at. The benefit of using Boons or Banes is that they modify the probability without making a task impossible.
So imagine our hero is trying to attack a baddie that's in total darkness. Could fire one pistol into the dark, but instead they choose to dual wield and fill the room with lead to increase the probability of a hit. Our hero has Gun Combat -0.
With -DMs due to darkness and dual wielding penalties, scoring a hit may be impossible. (-6 DM)
With Bane die, (or maybe multiple banes if you remove the restriction) a hit may be highly improbable, but never impossible.
And I think that's how you want things. Skills and maybe ability modifiers should provide DMs, but situational things like darkness or range are better represented by modifying the bell curve of the die.
Bringing in more Boons or Banes would mean less math during combat too. If I know my character has Gun Combat-2 and a +1 from his DEX score, I know I'm always adding +3 DM to my attack rolls. No need to consult tables, check character sheet or do more math. Choosing the highest or lowest pair of dice is easier that varying the math formula that needs to be solved to hit.