There are some visual novels in which the porn is essential to the plot (notably Saya no Uta, where you will find out near the end that sex with Saya was absolutely vital to her for spoilertastic reasons, as well as sex being used as a plot device on two different occasions).
But more importantly, do not lump Sengoku Rance in with the games where porn is just tacked on. Sex is a core element of Sengoku Rance (and the Rance series in general).
Sengoku Rance is a grand strategy game set in Sengoku-era Japan. The protagonist is Rance, who is basically a walking sex machine; he's a complete asshole and he only thinks about sex. He's on vacation in Japan to fuck all the beautiful ladies there, and he becomes buddies with Oda Nobunaga to the point where Nobunaga lets Rance act as the secret ruler of Oda, and Rance decides to conquer all of Japan in order to have sex with all the beautiful ladies.
The game is heavily story-based, with each country having its own characters, and many of your commanders having a unique look and personality and story events. You can spend an action to see such a story event, and in the case of female characters, this is occasionally a sex scene. Which story events you can get at any given moment depends on situational factors and the affection system, which is also the upgrade system, with commanders getting upgrades when their affection changes from 'normal' to 'trust' to 'love'. Sex scenes are also gained by conquering capitals of enemy countries and some other methods (clearing certain dungeons, etc).
Though the porn may seem like a well-done novelty at first, you'll soon notice that getting Rance enough sex is an integral strategic part of the gameplay. Whenever Rance has sex, his satisfaction goes up (the Sat. number between your national power and your money), and satisfaction is required to get game-wide upgrades (whereas affection gives unit-specific upgrades). These satisfaction upgrades are the most vital ones in the game, allowing you to gain huge bonuses, unique items or even vitally change the game, as is the case with the extra action fans. (At the start, you have two 'action fans', meaning you can take two actions per turn. Using the satisfaction system, you can raise this limit all the way up to five, making your turns 2.5 times as effective.) With the heavy story focus of the gameplay and Rance being Rance, you end up conquering Japan just to have Rance make quips about sex and pork all the girls.
Could you do it without porn? Sure you could, but in the same way you could do Europa Universalis on an entirely fictional map without anything resembling Europe - while the core gameplay could be replicated exactly, it would be missing its defining elements.
And as for whether it's good, let's just say this thread made me reinstall the game again and it's now 2 AM even though I have to get up at 7.