Well I've always wondered why the Intrepid class was the only class that we saw on screen that had moveable nacelles.
Well, two basic possibilities: it's something related to the other
ways in which the Intrepid
is unique, or it's not.
If the former, we have three unique things to choose from:
1) This baby can land.
2) This baby is exceptionally agile, at least at sublight.
3) This baby is exceptionally fast at warp.
Flapping nacelles might be related to any of these, or perhaps allow the ship to have more than one of the above when non-flapping ships have to make compromises between the properties.