Well, if Rose gets reunited with Jack after death (along with all the other passengers, apparently), that takes some of the dramatic tension out of the entire movie, wouldn't you say?
I mean, why feel suspense or empathy for anyone's tragic impending "death" at the sinking if it's really just an unexpected momentary midnight dip into the Atlantic followed immediately by a dreamy warm, loving eternity in Titanic's rotunda?
Talk about having your cake and eating it, too...
(And, yes, I do think Cameron intended that interpretation. No one ever accused him of not pandering to an audience's contradictory impulses.)