It seems little more than an annoyance.

Web pages have control over their links anyway. If you don't want people to be able to link off your page, don't put links in.

Of course having this Javascript prevents people from going to a different page - but they can still open a new window (Except in IE of course which by default locks new windows to the same page).

It is quite annoying, similar on annoying-ness to the old "window.close()" thing which existed in early browsers, is now fixed in all modern ones.

Slarty