Backstab is kinda fulfil that role of "rule of fun" too. And it's not like a guarantee success on the roll. Spell slots can be rested easily since you are not in combat? Don't you think ?
Well, considering one let you get advantage on almost any melee attack for free and the other lets you spend a spell slot to replicate the effect of a cantrip... yeah I don't feel as bad about it.
In tabletop D&D Charm Person is a very abusable spell, one that forces an NPC to like you and trust you as if you were lifelong friends. If you're an inventive player, this 1st level spell can be really powerful, allowing you to fast-talk the NPC into revealing secrets, making them give you money or treasure, or convince them to use their social or political influence for your personal gain, etc. So of course you want the target to get a saving throw. But you don't need to worry about this in a video game, since there is no dialogue option to demand the zhentarim give you all their treasure, at most it'll slightly improve your chances to recruit a trio of ogres, which you might've succeeded in doing regardless.
Personally, I think it's perfectly fine. You may think differently, and that's OK.