I can see the merits of both arguments - for opening external links in new tabs and for not. The problem we're faced with, though, is one of technology and one I face every day.
Personally, I use Chrome for just about everything. It's easy to right-click a link and select "Open in a new tab." It's also just as easy to ctrl+click a link to do the same thing. But not everyone uses Chrome ... and some systems are set up differently.
Middle Mouse Button
On my home machine, the middle mouse button, by default closes whatever application has focus. So encouraging users to click a link with the middle mouse button is a very bad idea. On my work machine, I reconfigured this setting to prevent accidentally forcing unsaved documents to close ... but not everyone will do that.
Every browser does things differently when you open new tabs.
In Chrome, my new tabs open at the end of the window (to the very far right) and the current screen maintains focus. I prefer it this way.
In Firefox, they open immediately to the right of the active tab, regardless of how many tabs I have open. I find this mildly annoying when I'm multitasking because I have to hunt down the tab later.
In Opera (according to my co-workers who use it anyway), when you open a link in a new tab, the new tab gains focus immediately. They prefer this, I hate it ... so they use Opera, and I just keep it around for testing.
Internet Explorer ... I have no idea what that browser does because I'm smart enough to avoid it :-) But keep in mind that, as much as we hate to admit it, a lot of people are still using IE6 and will continue to until Microsoft finally kills it according to their proposed 2014 schedule ... and IE6 does not support tabbed browsing!
Should We Open New Tabs?
Kudos to @Rarst for the article, but I did note the date ... the article was in reference to design flaws from back in the 90s before we had tabbed browsing. In the world of IE6, forcing all links to open in a new window is an evil, terrible, unforgivable thing to do. For those of us using modern, tab-friendly browsers, it's not nearly as negative.
There have been several times where I've accidentally clicked a link in a question while I've been writing an answer and, for one reason or another, I didn't get the "you have entered text" warning and was shuffled off to another page. Other times, I've clicked a link to read for context and accidentally closed the tab when I was done ... losing the ability to hit "back" to return to the question.
Personally, I would prefer that externally-facing links opened in a new tab by default so I don't lose my place on the SE when I come back. I know not everyone agrees with this, and that's the reason why every major browser treats tabs differently.
My vote, though, would be that internal links open in the same tab. External links should open a new tab if users are in a modern browser. If users are in a clunky dinosaur like IE6, clicking a link should set off alarms and sirens and make their eyes bleed until they upgrade.