As a web developer, I feel external links opening in the same tab / window take the user away from my site (or the site I'm building) so I generally always force external links to open a new tab. The goal is always I think, to keep people on your site as long as possible. Opening an external link in the same tab stops that - maybe from the external link they find something else they want to read / do and don't come back? That would suck.

As an internet user - I'll generally force a link (command click) that might look external to open in a new tab so I can come back to reading what I was reading without having to reload the page.