"Why? The nacelle forward ends are in rather prominent view from forward angles; a device for shielding the rest of the ship would not need to be blocking the path to the nacelles."
Though I'd like to believe that the main sensor-deflector generates a shield in a rather literal, circular sense, I'll put this aside for the moment.
If the forward ends of the nacelles were not
protected by the main sensor-deflector (like other parts of the ship) they'd be exposed to hazardous impact of stellar debris.
Of course, one may argue that these project a phaser-style disrupting field incinerating such objects on a collision course but the apparent energy output (visualized by the red glowing rotation effect) would appear to be higher than the energy you will gain from the incinerated stellar debris.
Again, I'd rather prefer the deflector function, but then, it would appear odd that these deflectors are so apparently different from the main sensor-deflector at the engineering hull's bow (compared to the sensors and/or deflector spikes of the Enterprise
If one were to assume the nacelles' caps to be antimatter reactors, the inevitable question would be how this one works and maybe I have a proposal, partially inspired by this screencap: http://ent.trekcore.com/gallery/albu...arkly2_761.jpg
(Dr. Phlox depriving the USS Defiant
in the ENT episode "In A Mirror, Darkly" II of its warp power)
While it's not abundantly conclusive whether the relays he's removing could be the sparks for a matter-antimatter reactor or the plasma back flow tubes of the reaction chamber, notice the (unlit) tubular frame 'support' which I assume to be antimatter injector tubes from the antimatter pod below ("That Which Survives") that go into the reactor chamber above.
If the nacelle caps had such tiny antimatter injector tubes (followed each by a matter injection tube and a matter-antimatter annihilation blast exhaust tube) there'd be an explanation for the glow and the rotation of the elements which would make the nacelle cap look like a 23rd Century combustion engine, in a certain manner of speaking, which I don't think is too farfetched considering the design was born in the 1960's and knowledge of theoretical, futuristic propulsion systems was still in its infancy.