Just my 2 cents. Most users are like me and pretty lazy too, and it may well cost having them decide to use the forum or install it.
If PunBB followed the big crowd without thinking about its choices, it would stick to "_blank" links, true. But it's not, and it's little choices similar as this one which brings better things later, because not using "_blank" is really the way to go. The web isn't a Win32 program, and those "_blank" link might not be this annoying for a common windows user (they are for the Win32 advanced user), but it is for everyone else. For example, I'm using an iBook besides other web clients, and I rarely use the trackpad on, except when I click on a "_blank" link, very annoying... This is just an example, of course.
The main idead is that the web wasn't designed to be able to get out of the window we've put it in (meaning "_blank" links, but also popups are bad interface design for a website, sorry for all those pseudo-designers who think it's leet to put a 800x300 flash in a 800x300 popup as a website, it's simply wrong), I'm certainly explaining it bad, but you should find enough talk about the subject on the web to understand why it's bad.