Others use closeness to the couple as a guide, or maybe some people give every couple whose wedding they ever attend a flat rate of $100 regardless of their relationship. Which of those is more fair? Is it fair to rank your friends and relations in a list from closest to furthest and have a chart with a monetary value based on their ranking? What if you like the bride, but not the groom - does the couple as a pair slip down the rankings in that case, and attract a lesser gift? What about the second system there. Do you give $100 from now until 2050, or index with inflation?
There's some level of ridiculousness in every system for deciding how much to spend on a gift for a wedding. I'm not sure why this particular system for guests to use when deciding on a value of a gift garners quite so much bile, as compared to the others. As hosts we need to understand that everyone has very different systems and amounts they can afford, and be grateful for any gift received, which they should be doing, regardless of how their guests decide how much to give.
I don't think there's a ranking system quite in the way you're saying, where if you "like the bride but not the groom" they slip in the rankings. It's more that you're more likely to give a larger gift to somebody you really love, and a smaller gift to somebody who is a passing acquaintance. So it might be quite normal for the parents to give their daughter a gift of $1000, but a sister to give her brother $200, and a man to give his best friend $100, and a woman to give her coworker $20. I mean, surely most people don't give exactly the same value of Christmas presents to all people they know, regardless of how close they are. They usually spend more on their immediate family, a bit less on friends, and there are probably plenty of acquaintances that they don't exchange gifts with at all. This is just how relationships
work, where you often exchange within closer relationships
at a higher rate. I'd be astounded and a bit uncomfortable if, say, a coworker gave me a $500 gift. Of course, my wedding was closer to $50 a head, I think.