I mean, if you can jettison the core ...
It would not be all that hard to think of other reasons to want to put some distance between the saucer and the lower hull.
If you absolutely had to land on a planet, and the lower was damaged to the point that it was no longer aerodynamic.
TWOK showed that the dilithium chamber can emit enough radiation to kill someone in minutes, a major breach could make a separation (for days or weeks) a real good idea.
I wrote a story once where the Enterprise Dee was having it's antimatter bottles refilled, and the ship separated for that. The saucer moved some distance away
Or, if the ejection system was simply "offline."