Increased Maintenance just sticks out because it's the only d-mod that renders ships unusable if you don't have two specific level 3 skills.
I feel like the impact of IM is way overestimated. It's 100% more supplies per month, so based on the *worst* parameter, it costs as much as having another ship of that type. However, this is offset by:
1) A 20% reduced deployment cost with no combat penalty; this alone can go a long way towards mitigating the supply use, depending on how much you use the ship
2) No increase in fuel consumption, compared to having a second ship
3) A 50% increase in crew required, vs 100% for a second ship
The downside is, of course, not having the 2nd ship. I mean, it's not a net benefit, but then again it
is a d-mod. IMO it's nowhere near being unusable.
The pool of d-mods is small enough that you will encounter any given d-mod very frequently (e.g. a combat ship without hangars with 2 dmods has a 57% chance of having increased maintenance and/or erratic fuel injector).
For a non-phase non-carrier combat ships, there are 9 d-mods to pick from. The first time we pick, there's a 2 in 9 chance of getting IM or EFI. Assuming we didn't get either, the next time we pick, it's a 2 in 8 chance. Thus, the probability of not getting either of those for 2 d-mods is 7/9 * 6/8, or around 58%. The chance of getting one or both of IM/EFI, then, is 42%. Unless I've miscounted the number of applicable d-mods? Or messed up something else?