That's a "slug", it's extremely common and IMO a good thing.
Sure, you can create a weird looking or even misleading URLs that way but I don't think it's a big problem because 1/ as soon as the page load the URL gets rewritten to the real title and 2/ it's often very easy to obfuscate links regardless of that. Many platforms allow you to hide your links behind an href with some markup for instance, so you can make bogus links very easily. Think of something like:
<a href="http://evil.org/">http://google.com</a>
This is very common in spam emails.
You can't even trust the browser's link preview tooltip because it can be overridden in JS. So in general it's a bad idea to blindly trust an URL "from the outside", slug or not.
I really, really wish youtube would do the same thing for instance, it's completely impossible to know what a youtube link is pointing towards. You could argue that they want short URLs but since they already have a "youtu.be" shortening service to make them even shorter it feels a bit redundant.
I'm talking about the "preview" usually at the bottom left of the browser when you hover a link. By using a Javascript event handler on the link you can override what happens.
Google does that for instance, if you hover on top of a search result it'll look like a direct like to the website, however if you look at the HTML source it looks something like this:
So even though the href goes to wikipedia in this case if I click the link the browser goes to a google page that then redirects me.
You can see the real URL by right-clicking on the link and then hovering again, it causes the "onmousedown" code to run and replace the href by the real value.
Duckduckgo uses a "click" event handler instead. As far as I can't tell Bing doesn't do anything and directly links the target website, which is odd. I may be missing something.
I don't know how this works in different frameworks, but in principle, nothing prevents you from arranging it so that /user/6380/hanselman is valid but /user/6380/[somethingelse] 404s.
Edit: but this has the potential to break links if the user changes their display name.
I use the same scheme for blog posts on a website I develop. I solve this problem by looking at the slug and redirecting to the correct one if it is wrong.
"Other people do it," by itself, is not a great justification.
It's yet another way to mislead. Just because other misleading schemes exist doesn't mean this isn't also misleading and potentially bad.
As for "How so" ... I didn't think it through. I'll go with "potentially not good," but equally not thought through. Since the subject of the article is URLs as UI, when you send someone a URL to "look at this", what they see is the URL, and in my example the human readable part is "the site" and "a3n", but what they get is nothing to do with a3n.
I can only intuitively start with "that's misleading," and imagine (but not point out) the possibility of "something bad". Maybe something merely annoying like rick-rolling.
> The only thing that matters there is the 6380. Try it https://stackoverflow.com/users/6380 or https://stackoverflow.com/users/6380/fancy-pants also works. SO will even support this! http://stackoverflow.com/u/6380.
This works too: https://stackoverflow.com/users/6380/a3n
That doesn't seem right, and on a different site could even be dangerous.