I think the imaginary Jadzia from If Wishes... is an excellent demonstration of why it just wouldn't have worked. Would have made crap TV too...
You know, having rewatched the episode the other day, that thought dawned on me, too. Jadzia is nothing like what Julian wants her to be. He's actually in love with his imagination of her, and not the real Jadzia. That's another reason why I never thought they'd make a good couple.