Since we have sites and spend a fair time using the net, we forget other people aren't all familiar with their browser's features. So really either way you're forcing users. I can't stand external links that open in the same window. A user may want to have both windows or tabs open, etc. Unlike news sites, forums generally want members to stay longer to post and interact. Opening external links in the same window doesn't help that.
I think it's foolish to think most (or all) people know that they have a choice or enjoy having to have an extra step when wanting to open a new page.
It's nice that IPS added this feature, but I think it was a mistake to have it enabled as default.