Why would you want to "win it back" in the case of yourbrand.xxx? It's worthless to a non-porn brand.
"My concern is that many people on HN are building brands, and the last thing you want is someone buying yourbrand.xxx and covering it with porn."
Why would this be a concern? It's not worth anything to a pornographer to, for example, register virtualmin.xxx and cover it with porn (except maybe for extortion value). I could see "google.xxx" or "twitter.xxx" being a problem, but in those cases, they'd want to sue for trademark infringement. The domain name is irrelevant.
"If you have an established brand, you almost have to buy the .xxx."
If so, it's a license to extort money. I'm just not going to participate in that sort of scam, and I don't think anyone else should, either. All the people that do are funding further efforts in the same direction.