Tactically that would only make sense if the engineering hull could keep an enemy ship entirely occupied, if there was more than one enemy ship engaging then its going to be difficult to keep them all from pursuing the saucer.
You are right though, if you did suffer a warp core breach you aren't going anywhere quickly. But in almost all situations help is only one subspace call away.
Huh? No way; whether you're in a battle with 3 cloaked Romulan warbirds, or have an imminent warp core breach, you have minutes, at most, to get help. Other starships aren't that close by.