There are users that have a domain name as their user name, are such user names allowed?
I'm not seeing the downside.
If a user is posting good content, who cares what their user name is? If you're implying some sort of spammy-ness to the name they choose, I'm still not seeing the downside. They still have to post good content to get their name in lights. That's a net win to us!
The identity you choose is a personal thing. If someone wanted to be called (for example) "Ubuntu Fanboi", we wouldn't be having this conversation. So is a username like "AskUbuntu.com" really all that different? It's not indiscriminate advertising... it's a username; and it's not a part of the information content of this site. In short, there is no "spam" to a username.
Users have three major reasons to contribute to projects like Stack Exchange:
If that purpose happens to be a bit of self-promotion, we all like to show off a bit. So, we provide two places that allow you to establish your self-identity: Your username and 'about me' profile.
If we take away that little bit of self-identity, you're also removing one of the major purpose-driven motives for using this site… and that would be a disproportionate reaction to allay some momentary irk you assign to something so inconsequential.
The obvious concern is spam, which I think it quite reasonable since we don't allow signatures nor disclosing affiliations except when promoting a product. After all, if you can't post a link or company name at the bottom of your posts, why should you be able to put one in your username which appears at the bottom of each post?
You could argue that the latter case is ok because there was going to be a username displayed anyway, so it's not adding any clutter, but it can still feel pretty spammy.
However, I haven't really noticed it being a problem so far, so I think it can be handled on a case-by-case basis for now. Even though allowing them seems to be inconsistent with the existing anti-spam policies, it doesn't look like it's something worth worrying about right now. It seems like it could be impractical to enforce across the board as well. If you see one that looks really spamming you can post about that specific username here on meta and see what people think (not sure if flagging for moderator would do any good). In particular, if it's a website for something not suitable for all audiences, the team may want to do something about it, but that's probably true whether the username is a website or not.
Your name is your brand, whether or not it is your birth name, a stage name, a company name, or a URL.
Should people who do business under their real names be prevented from using that as their display name? Do we make you change yours if you decide to start running a business called "Tom Wij's Information Services?"
Furthermore, what's to stop me from building up a ton of rep under the Aaronaught/Aarobot name and then starting a business with that name/domain? Does the name then suddenly become taboo?
As long as a name is actually a name and doesn't represent obvious hate speech or propagandizing, and as long as they aren't actually posting spam, then it should be, and is, allowed.