Plus in "Twilight" Enterprise was able to keep going with only one nacelle.
If I remember correctly in "Year of Hell" part 2, Janeway asks about engines and B'Elanna reports that they haven't got engines yet because one nacelle is offline and the other is a lost cause. You could take this to mean that even although one nacelle is totally destroyed, the can still go to warp although probably with less efficiency and speed.
Thanks guys, I didn't know any examples, it was only something I had hoped was right.
The whole thing with 3 or even 4 nacelles is to give the ship a stronger warp field in order to be able to hold a generally higher warp factor over short distances. The trade off isn't with mass here, but with max range. While two nacelled ships can cruise at smaller warp factors than 3 or 4 nacelled ones could, they have a natural longer range due to the higher fuel consumption of the 3 or 4 nacelle ships needed to run the extra nacelles. So it's the speed vs. range tradeoff thing. Also, 4 nacelled ships can warp with only 2 nacelles and keep the other two for emergency situations or as a backup, but their nacelles need to be on the same distance from the ships center of gravity to be able to run simultaneously (meaning that for example the Yamamoto class TMP battleship couldn't use all 4 of her nacelles at once, since the bottom pair is further back from the center of gravity than the top pair). Also, if we go by Gene's rule, a single nacelle ship should have rows of warp coils in her nacelle.
With all the same engines, four engines would only need to run at half power to equal the same power drain as two engines at full power.
Keeping two nacelles as spares in a four nacelle setup could be tremendously wasteful. Considering Voyager's landing gear setup, its nacelles have to account for roughly a third its mass in order to counter balance the saucer, as the backstage explanation has no confirmation within the show. In "Timeless" Voyager's saucer also stays above the ice with the nacelles level as if counterbalanced by the nacelles as it slides to a stop. After all, the coil segments of the nacelles are gigantic metal arcs. It's also the only way certain designs can have centers of gravity centered on impulse drives. Not that that is much of an issue considering "All Stop" orders and instant reverse thrust indicates an asinine thrust vectoring capability.
I don't see how being equally away from the center of gravity actually matters considering the high placement of the majority of nacelles in two nacelles designs. If there were a balance issue with the mass, then two nacelles both above the center of gravity should cause the ship to constantly nose forward and down. Wouldn't that make designs such as the Challenger, Norway, Galaxy, and Defiant the only practical ones? Unequally placed nacelles, such as the three nacelle Excelsior kitbash should only result in a warp field shape similar to that of a ship with an engineering section, going by what has been seen of the Galaxy class' radiating warp field diagram. There is also the Breen ship which is horribly imbalanced.
If we go by Gene's rules, the Defiant, Peregrine, Saber, Oberth, Data's Mission Scout, Sydney, Challanger, Curry, and Denube, and for that matter just about all shuttles would all be physically impossible because their nacelles cannot see at 50% of each other. Voyager can fit there too, because in the down position it wouldn't work, but in the up position, even though the nacelles are visible across from each other, I don't think the Bussard collectors can fully see past the saucer. The dual-row-coil-assembly-in-single-nacelle explanation should only matter if all of the Roddenberry rules matter, but they've never been strictly followed. Even the Sovereign breaks the rules thanks to its whole nacelles not fully clearing the saucer, only the Bussard collectors clear the saucer. Single nacelle flight by paired nacelle ships also completely throws out the concept of needing dual row coils in single nacelles, if the above examples hold out.
As rules of thumb for designs they're decent and can lead to some interesting designs, but it seems apparent that the universe follows the rules as nothing but generalities and not law.
