It might also be worth to mention that using target attribute is not xhtml valid. I do usually open links in external window or tab because I see that most regular users (not the advanced ones) want it that way so that they can always get back to the site they were on - usually they would go deep into the other site and then it become unfriendly for them having to click back multiple times.
So in terms of usability I think that there's more users that don't use special techniques to manually open links in new window/tab.
With regards to the xhtml validation, you might want to decorate your links with rel="external" or some similar word then use this JS function to handle new window open. I did it like this 99% of time in the last few years.
function externalLinks() {
if (!document.getElementsByTagName) return;
var anchors = document.getElementsByTagName("a");
for (var i = 0; i < anchors.length; i++) {
var anchor = anchors[i];
if (anchor.getAttribute("href") &&
anchor.getAttribute("rel") == "external")
anchor.target = "_blank";
}
}
/**
DOCUMENT LOAD
**/
$(document).ready(function () {
/**
external links
**/
externalLinks();
....