Still favor multiples, if you have just a single reactor/warp core and it gets knocked out some how, why the entire ship would be helpless. What kind of moron would design a ship like that? Oh wait, yeah.
Well, let's consider a modern-day example... a nuclear power plant.
We refer to this as a single facility. And on rare occasions, such facilities consist of only a single reactor. But more often, there are multiple reactors which make up the facility. And those reactors making up the facility may not be located very close together... what makes them a "facility" isn't proximity, it's the fact that they're all centrally controlled and coordinated.
All we know for certain is that in some circumstances, during TOS, we get a clear reference to a "facility."
That is in no way evidence that there is only a single "reactor" at any one location in the ship... just that there is a single "reactor system" (meaning common "organizational structure").
The "row of tubes" in Engineering could be a string of m/am reactors. Or there could be a single big reactor under the floor. Or there could be rows and rows of reactor chambers in the nacelles (as TAS showed us, and as I treat things).
There is ZERO HARD EVIDENCE EITHER WAY. After all, the ship was never actually built...
For me, I go back to MJ's intent... the "aerospace model." CRA and others support the "automobile model" where the nacelles are the tires, and the "engine" is elsewhere in the vehicle.
But... in that automotive model... you talk about "the engine." Think about the engine in your car, however... it doesn't have just one cylinder, does it? It has four (if it's a little puttering golfcart of a car!) or six, or eight, or (if you have a high-end sports car) maybe twelve.
Yet with all those "reaction chambers" (and that's a very appropriate term, by the way) in that engine, we still call it a single engine, don't we?
One, you have multiple fusion reactors to act as a backup.
Except that the whole point of m/am as a power source is that it produced a lot more energy per gram of reaction mass than fusion possibly can.
Two, the antimatter reactor is deep within the secondary hull, with safety systems out the wazoo, and an engineering crew on hand to keep things running smoothly, and thus very well protected.
Which is why we want our nuclear reactor power plants in the middle of New York City or Los Angeles, right? They've got "safety systems out the wazoo, massive engineering crews, and are very well protected, after all.
Think that's a good idea? No? Why not?
Now, if you want to make your ship easy to disable and/or destroy, go ahead and put the reactors up in the nacelles, and see what happens the next time you come across a Klingon ship with a wiseass gunner who wants to prove how good a shot he is (and before anyone brings up ST III, I'd like to remind folks that the problem there was that Kruge ordered the knucklehead to "target the engines" to disable the Grissom and instead, he had his "lucky shot" and hit something else, thus destroying the ship).
If you have shields and deflectors... how is having them in the nacelles making them any more vulnerable than having them in the main hull?
Further, if you DON'T have shields and deflectors, how is having them in the main hull making them any more secure and protected than having them in the nacelles?
Do you think that the shielding on the nacelles is, for some reason, less robust than that over the rest of the ship?
Do you think that an unshielded secondary hull is going to provide any more impediment to a full-power weapons system than an unshielded nacelle? I think that the term "hot knife through butter" is probably appropriate...