Well, there's some truth in that. I'm not sure. But if many people fix their browsers to ignore those attributes, than more sites may use javascript to open links in a new window (some sites do that now too), which is even worse than the current situation. Surely, you can block those pop-ups easily, but then the page doesn't get loaded anywhere, it's not just that it's loaded in the same window.

(Also note that there's also a better reason of using the target attribute: loading pages to the correct frame on framed pages. It's only the case when it opens a new window that should be ignored.)