Late game they are affordable, but early and mid-game they're redundant when you can tech/spam III/II bombers that are far more effective at destroying structures instead of waiting for your dreadnoughts to clear the planet. Increase in damage bonus vs structures would fit with the '12 speed' persona.
If dreadnoughts could actually destroy structures with greater rate, I would consider them; however, as it stands, they are not useful. If we speak about the bonus of engine damage, obviously riot starships are better suited for that type of task, and have similar range, though of course not really helpful against structures.
