Would it be dangerous to not have the sac removed after having a rupture of a saline breast implant?

No. The breast forms a capsule around any implant.  If this capsule gets thickened, hardened, or calcified, it should be removed (capsulectomy) and the implant changed.  But that is NOT the same as a saline implant merely leaking into a normal, soft capsule.  The saline fluid that was placed was sterile, and unless there is a concern that it was contaminated, there is no reason why the capsule should be removed.  Removing the capsule is not without risk, particularly in a thin, small breasted woman.  Dissecting the capsule away from the breast can cause contour irregularities, and increase risk of postoperative hematoma or seroma.  

The shell or sac of a saline breast implant is made of soft rubber that was sterile at the time it was placed.  It seems no more dangerous to leave an empty implant than a filled implant.  That being said, the empty device is flat and has "sharper" edges and/or folds that can be irritating to patients over time.  In addition, one deflated implant tends to look odd and asymmetric.  The surgery to replace a deflated implant is usually much less extensive than the original surgery, as long as there are not other factors present, like capsular contracture.