Strongly disagree with this. It's fairly trivial to configure this behaviour in your browser (a plugin in the worst case), and really not much of a burden to hold shift when you click your link anyway. The cost for everyone else is that there's no easy way to get "new window" links to open in the same page, which is particularly frustrating on mobile devices.
How does that break usability? Reddit does it. Also HN is an aggregator, so it makes sense to keep it open in a tab and open the links in new tabs.
It's actually a preference on reddit, and one that (IIRC) is off by default.
In terms of breaking usability, people expect links to open in the current window/tab. Having them open a new window/tab is disorientating at best.
Because others do it doesn't mean that everyone should do it.
If you want to open it in a new window or a new tab, just click with the correct key on your keyboard pressed.
That's what people expect and that's the correct way to do it.