That rule breaks down if it isn't qualified. What if the way they want to be treated isn't good for them? Especially in the case of immature people (and especially immature children). A simple example: always giving a kid a ton of candy whenever they want is hardly treating them well.
How we should treat others is rarely an easy-to-discern topic. Lot of room for error there. We could say, "oh, but it's assuming they know what they're doing; or, oh, it's assuming they're a normal functioning person; etc". But the fact is, in life, a lot of people don't know what they're doing. They struggle with their relationships, their finances, and their very purpose of life. Whether it's a high school sophomore desperate for friends or a successful, rich guy with a mid-life crisis, we all have moments of weakness where we make irrational decisions and have irrational desires.
Building on that point, further complicating matters is that people often don't know what they want, often including how they want to be treated. Oh, we know it usually means that people want respect, love, money, happiness, whatever. But what does it mean at a tactical level that actually gives us the insight for how to treat them well? Was my mom really jealous of me getting a box of cookies for a friend instead of her (long story), or did she just wish we could hang out more together before I left again for my new home? Well, turns out that although she was jealous of the cookies, she didn't care about the cookies at all. She wanted to spend time with me. Now, how is a normal person who sucks at conflict resolution supposed to pick up on that? It took me 30 minutes of talking with her.
I really think that this rule is useless, unless one is some kind of prophet who can see into people's innermost hearts. It offers little real-world tactical advice. You might argue that the Golden Rule doesn't either, but I'd say it's much easier to implement, and much more difficult to screw up. And as the other guy notes, this Platinum Rule would derive from the Golden Rule anyway.