As a web designer I try to stand on the same side as the users as much as I possibly can. I'll always fight for the their best experience, it's the users we create websites for after all.
One of the battles that I've been fighting with clients a lot during the years is the dilemma about how external links should be opened.
As quickly as there is a link that points to an external website most clients wants to open the destination of the URL in a new tab/window. This is terrible usability. Why? Let me explain.
The first problem with this approach is that it breaks navigation. For most people the back button is used very frequently, it acts like a lifeline always providing a way back to safety. This happens to me a lot: I open a link to an article somewhere, after I'm done reading I decide I want to get back to the previous page. I try to back up. Nothing happens. I try again, still nothing. Why does the browser not respond to my actions? Has it frozen? No. Ah, it's only that I'm not in the right tab anymore... This is annoying as hell.
The back button is sacred. Do not mess with it.
Another problem is that it looks really bad. It feels like a poor attempt from the website owner to desperately try to hang onto the user, even when it's ready to leave. Try to imagine yourself as a customer walking in to a store. You're looking around for a while but then you decide to leave. When you're out on the street again you surprisingly find the salesman from the store on the ground grabbing hold of your foot as you walk. That doesn't look good, right? I've actually never seen this in public but I always get a bit suspicious when it happens on the web.
Let go. It will pay off in the end.
One more thing. As I mentioned before, sometimes the user won't even notice that they've entered a new tab, meaning they're keeping the session active for the old website without the intention of doing so. It will inflate your analytics and present you with irrelevant numbers and inadequate facts.
Not good, eh?
So, all of this ends up in a crappy user experience and bad usability. It's very important to let the user have a choice. Some have a preference to open links in new tabs and they do so by holding down the CMD key, while some want the regular standard behavior. Don't try to change that.