I don't know about that. I'd say the removal of the crystal from engineering isn't removing the power- it's removing the use of the available main power transfered from the nacelles. The crystals in each nacelle m/arc would no doubt be equally important, but remove the one from main engineering, and you're left with having to use the back up reactors that have significant less power because they're probably not m/am. There would be something with the way the systems are integrated that removing that main engineering crystal takes away the use of the warp engines despite their independent m/arcs. But how and why would become the question. One point I make would be that it Starfleet ships being borded must be incredibly rare, and getting past security to get to engineering and remove that critical crystal would be highly highly unlikely, despite the Enterprise getting disabled so bloody often. Now I say they would be a safer type of energy producing reactors because the main point of having the nacelles so far from the ship was partly due to the dangerous nature of the m/am reactors in them- and possibly due to the warp engines in whole themselves.