I'm pretty sure Rick Sternbach has stated that the real-world reason Voyager had moving warp nacelles is that the producers wanted the ship to have visual interest and so wanted something on it to move, so he came up with the idea of variable warp geometry and the idea that they would indeed move tiny amounts while the ship was in warp. In his original designs, the field windows on the engines would hinge outward. There are sketches floating around somewhere that Mr. Sternbach did that show the Voyager in a form very near its finished form with fixed-wings and the nacelles hanging slightly below. I guess the producers liked the folding result since John Eaves even played around with incorporating this into his design for the Enterprise-E but it was ultimately dropped.
Personally, I like the idea that the warp field is already forming when the nacelles fold, and that it's the 'squeeze' that affects the ship's acceleration abilities. I do agree that it's probably something that made the Intrepid class special that wasn't useful enough to incorporate into other ships.
