I've inherited lands a number of times, although it's still a pretty rare occurrence. I myself haven't noticed Caravel's observance that there may be a link between happiness and provinces joining your kingdom; but he could very well be right, since I've never paid particular attention to that aspect. Even if maximizing your peoples' happiness matters, though, I still doubt a player's chances of inheriting land from a defunct faction is greater than 10% or so. Unfortunately, it just doesn't happen that often.

Also, just to clarify: You won't inherit lands by marrying off your princesses--you'll only inherit lands by marrying your sons to princesses of other factions.