If you want to get technical...any saucer section ship, even the Enterprise from the Orignal series could separate. I have technical manuals from the 70s way before TNG was even conceived and it showed where the ship can separate. It was just a lot easier to do it by computer graphics than breaking their models apart in the 60s and there were no civilians to worry about either to use the saucer as an escape pod.
Only against the Borg once was the TNG Enterprise saucer used for an extra attack, and that was mainly as a distraction to sneak the shuttle in. Losing the saucer means you are losing the warp core power to the saucer, so your weapons will not be as powerful, your phasers are on the saucer, so now your battle section only has photons in front or rear phasers. As combat goes, the separated ship is a disadvantage. All it was designed for was to separate to get the civilians or bulk of the crew in a no win scenario out of there and the battlle section to slow persuit to allow the saucer to get away on impulse.

I believe they've stated the reason for not allowing the ships to split is based on Starfleet protocol and not game mechanics. As I recall, the reason some of the ships can seperate is to keep the non-essential crew safe in extremely dangerous situations or something like that. Believe it also acts as a giant escape pod in dire situations where it's clear that there only way to save people is to split the ship apart and have the saucer run interference until the rest of the ship can warp out or part of the ship is so damaged that it can't be saved.

I highly doubt any of those split ship maneuvers that have happened on the show are Starfleet approved tactics and is probably something that almost never happens.

I could be entirely wrong, but that's what I recall being told by a friend of mine who's really into Star Trek. I enjoy the shows and movies, but I'm not one of those people that memorizes all the details.

Edit: Takes me awhile to write posts up, heh. Galvon said the same thing above me, so guess I did remember correctly.