Of course, the rule isn't part of the Star Trek universe. It's only part of the production history of Star Trek. And that's ancient history, really, long since rightly forgotten.
The whole idea that only certain limited ways of achieving warp would be acceptable is ridiculous to begin with, when we consider all those alien vessels that don't have any engine nacelles at all, or identifiable engines of any sort, or perhaps not even a hull. Warp is easy in the Trek universe once you get the hang of it, and anything goes after that.
Timo Saloniemi
The whole idea that only certain limited ways of achieving warp would be acceptable is ridiculous to begin with, when we consider all those alien vessels that don't have any engine nacelles at all, or identifiable engines of any sort, or perhaps not even a hull. Warp is easy in the Trek universe once you get the hang of it, and anything goes after that.
Timo Saloniemi