Back when TNG was about to be released, a great deal was said about the ship's ability to separate the battle section etc. To me it sounded like another boast that TNG would have better special effects (among other production values) than TOS.
Never made much sense to me. One or both sections have to lack some important system (how about a warp engine for the saucer?), or in "normal" mode, have to haul an awful lot of redundant equipment. It could be argued it's easier to have two complete spaceships.
Then there's the battle section looks very weird IMHO.
Turned out that, after the show-off in the pilot episode, that capability was used only in a few episodes and, I dare to say, never in a decisive manner.
I read somethere the designer of the Enterprise D wasn't told the ship was supposed to separate until after the design was complete (and with not much time left to devote to the problem).
I wonder if some more aesthetic solution could be invented given more time, or if the shape of the ship would have to be greatly modified.