At this point you then have to come up with a reason for the saucer NOT to be equipped with a warp drive of some capacity. In the Star Trek universe, every vessel above the size of a work-bee gets to have a warp drive. A two person shuttle pod is so equipped. What possible reason could there be for the designers of the saucer to make the conscious decision to exclude a warp drive? I'm not talking about some monster warp nine plus propulsion, but something in the warp three or four realm. Starfleet does seem to favor external nacelles, however they are apparently not a requirement, many of the non-Starfleet ships we see do not have external nacelles. As a emergency only propulsion, enclosing a warp drive internally in the area aft of the main shuttle bay may have been deemed sufficient. As noted above, something the size of a two person shuttlepods has a warp drive. It would seem to me, given missions of long range exploration out in the Federation's "boonies" and beyond, that the Enterprise's lifeboats would all have a warp drive. Depending on where the abandoning of the ship occurred, rescue might be years in the future. If the first can have a warp drive ... why not the second?