My first opinion is it's break the separation of **content** and **behavior**. and new link in new window will make browsing difficult for screen reader user. 

I personally do not like to open web content pages in window, but if client want then we will have to do. 

**From usability point of view**

> consider
> the following two situations where a
> user doesn’t know upfront if the site
> opens links in new windows or in the
> same window:
> 
>    1. user wants to open link in a new window, but the site opens links in
> the same window,
>    2. user wants to open link in the same window, but the site opens links
> in new windows.
> 
> In the first situation users can
> choose to open a link in the new
> window using context-menu or shortcuts
> described in the next sections of this
> article. In this situation users are
> the initiators of actions as they
> decide how the linked page should be
> displayed. Here site’s behavior meets
> user’s expectations resulting in a
> **good user experience**.
> 
> In the second situation users would
> simply click on the link and suddenly
> find out that the link is opened in a
> new window. In this situation users
> are the responders of actions as they
> need to react on the way how the
> linked page is displayed — for
> instance close the windows which was
> opened automatically. Furthermore,
> here site’s behavior doesn’t meet
> user’s expectations resulting in a
> **bad user experience**.
> 
> Users find it annoying when the site
> does something without asking them to
> do so. If users want to open new
> windows let them do so and don’t
> indulge their intelligence by making
> decision for them otherwise. Don’t
> force a new window upon users unless
> there’s a very good reason to do so.

reference - http://www.smashingmagazine.com/2008/07/01/should-links-open-in-new-windows/

**and** 

Believe if or not, there are instances when using a new window is a good idea, such as when:
   

 - The link is for a document, such as a
   PDF or Word file. Opening a new
   window will allow the image or
   document to download in the
   background. It also prevents users
   from accidentally closing the browser
   window when they close the document.
 - The link is for a large image. In
   this case, a new window allows the
   Web user to keep a browser window
   open while the image is being
   downloaded.
 - The link is for a printable version
   of an article or Web page. Here, a
   new window allows users to keep the
   current window open while they print
   the article or page in the
   background.

Source - http://articles.sitepoint.com/article/beware-opening-links-new-window