I can remember the early pre-view drawings of Voyager in TV Guide before the show came out with the warp nacelles fixed in a down position. I truly wanted to see the nacelles move according to the different speed of the ship.
That's really the point. We know why it was really done... for the same reason that so many other nonsensical design decisions have been made through the years... "Becuz it'll be kewl!" But is there any plausible technical reason for doing so?
I can think of quite a few technical reasons for NOT doing so... not the least of which are the decreased mechanical strength of the nacelle attachment points and the increased complexity of the hardware required to go through hinged joints.
But is there any plausible reason for moving the nacelles, other than because Jeri T wanted it that way?