Since we know that the Feds can make just about anything go to warp, including near-derelicts and mining rigs, it is only logical that the saucer could have warp, too.
That ignores what the production was actually allowing most of the time.
One of my pitches in '90 involved installing deployable warp nacelles in an intentionally separated saucer, or installing unseen means of getting the saucer to warp independently, in order to draw in a group that had been preying on other separated starships.
I was told very explicitly that it doesn't work that way, that the saucers can't ever go to warp, that there is no way to do this (not as a production-wise issue of expense or effort, as an in-universe story call), on to the next.
Of course I guess if Rick Berman pitched the idea it might have gone down differently ...