There has long been a disagreement between those who think that all links leading offsite should open in a new window, and those who think they should open in the same window. I maintain that this is not a real controversy.
Usually, but by no means always, those wanting to open a link in a new window are the owners of the website, and those in favor of using the same window are standards-supporting web designers.
The usual reason for favoring a new window opening is that the owner of the website is afraid of losing viewers. If the link opens in the same window, then the viewer has left his website. Of course, this does not mean that the viewer is gone forever. To my knowledge, there is no evidence for or against this.
The reason for favoring the same window is that strict adherence to web standards requires not using the target attribute of the link to open a new window, as this attribute has been deprecated.
In addition to this argument, the web designer usually points out that the viewer has two ways of dealing with an off-site link. He can choose to just click it, with the idea of using his BACK button to return to the original website. Or he can right-click the link and choose "Open in a New Window".
Why there should not be a "dispute":
To the best of my understanding, the reason for the W3C ban on the target attribute is that HTML – the language they are being "strict" about – is supposed to contain only information about WHAT is to be shown in the window of the browser. It is not expected to concern itself with HOW or WHERE it is shown.
HOW the information is shown is dictated by CSS. Interactions between the viewer and the browser are governed by the DOM. So consequentially, opening a new window should be done by scripting.
This is why there should be no controversy. If the site owner wants offsite links to open in a new window, then the web designer can do so by using scripting. This will not violate the "strictness" of the W3C, and the webpage will validate. See the paragraph below for an easy way of accomplishing this.