I always saw them creating a bond between Kahlan and Cara that was as strong as it was because they wanted to draw away from the kind of triangle that was created when Cara came on the scene. Plus, Kahlan really had no female friends or companionship outside of her sister, and the other Confessors, so it was interesting to have her develop a friendship with someone she wasn't really predestined to like.
There was something more complicated about Richard and Cara too.
Like you said, the connection they shared wasn't just that he was the Lord Rahl (at least not in the show), though that is how she tried to play it off. I think Cara was intrigued by the fact that someone, anyone, would stand up for her, and grow to care about her at all.