... at least, mostly.
I'm not an Objectivist because I do not accept that she proved an ought from an is. However, I recognize her brilliance and the fact that she got at least 90% right. Her major contribution is far more important than anything else in the history of philosophy, outside of Aristotle's work.
Regarding marginal utility, it has been awhile since I read her book on Capitalism. ("The Unknown Ideal," I think.) I would be shocked if Ayn Rand had no idea of marginal utility. She was, after all, a laizze-faire Capitalist. She understood economics. She certainly understood the concept of marginal utility.
If she didn't write about it a great deal, it is probably because it is understood that one's long-term rational self-interest will include one's calculation (mostly at the subconscious level) of whatever utility one gets from a particular action.
Marginal utility is useful if we are talking about economic concepts, but not so much if we are talking about emotional nurishment.
One virtue Rand believed to be important was honesty. Holding up honesty as a virtue is in one's long-term rational self-interest. This does not mean it is 100%. If someone says they want to know where you wife is so they can kill her, you lie. Her value to you is more than telling a lie.
But in general, it is best to be honest in your dealings with others. Now, how do you square that with marginal utility, as you seem to be implying that marginal utility is something the be-all, end-all of how to live one's life?