I personally prefer to open external links into a new tab/window, however I intentionally do so by opening that link with middle-clicking my mouse. While I do see why it can be helpful to others, I don’t very much like websites forcing me to open something in a new tab/window when I wouldn’t have needed it. It’s easier for the user to manually open to a new tab/window. However a user can’t (as far as I know) specifically influence to open a link in the same window.
So, personally, I would agree with @van’s reasoning for leaving the default as it is.
As for the topic of finding the option in the Preference section. I don’t know, maybe it doesn’t get noticed by everyone? Personally I have a habit of specifically exploring the settings of a website when I register but I guess not everyone does that. Maybe it’d be an idea to add a sentence to the Welcome Message, generally linking to the Preferences so every new user gets another nudge to look at it.