As a much needed addendum to my ramblings above I will briefly note that I do realize what the answer is. The nacelles are the seat of warp power, and the shuttlecraft doesn't have warp, and that saucer isn't related to what makes warp in any way at all, so it has to rely on simple old rockets. There. If you ever needed evidence for "self contained nacelles" that aren't linked to anything in the main hull, that's it. Because, once you hook the nacelles to that rest of the ship, all shit breaks loose and everything goes awry and before you know it, shuttlecraft can go at warp and saucers have antigravity drive and... I'm really glad this Halloween candy is almost gone.