Thongs are not quite as useful, but have definitely become a staple of western society. In fact, some may argue they are less useful than regular underwear or panties, but their "purpose" is to look attractive and make the wearer feel sexy. They are most often worn by teenage to middle-aged women.
Here's a great idea: Let's combine the two!
Presenting, the Thong Diaper:

Perhaps it's actually a Diaper Thong. Honestly, I think I'd rather not know.
Let's break down the patentability of this.
Non-Obvious - A diaper that's also a thong? I've never thought of that before. Have you? I think I'm gonna give this a pass.
Novelty - I don't think anyone has ever thought of this in their life (well, clearly SOMEONE has). Another pass.
Enablement - What industry does this belong to, the diaper industry or the thong industry? Honestly, I doubt it could compete in either. That said, it seems reasonable that any professional in the diaper industry could figure out how to reduce the size and make this thing. Not sure about the thong industry though. I'll give this a Maybe.
Usefulness - This one is the real stumper. What the hell is the point of this thing? Is it for babies who want to feel sexy? Is it for pedophiles who want to dress up the babies *shudder*? Is it for women whose partner has a diaper fetish? Honestly, the only possible legitimate use I can think of for this is for seniors who may want to feel sexy wearing a thong, yet can't control their bowels. Mmm. But even that's a stretch. I'm gonna go ahead and give this a NO.
So seriously, what the hell is the USPTO up to these days? Does someone there have a diaper fetish? I'm glad our tax money is being well spent.