Noted : advantage: viewers won't go away from your site is the basic reason. They can still return to it and it's still in a window. Disadvantage I would imagine it annoys many people to have someone else control their browser.
Personally, I hate it if I am taken away from a website I was busy with. Within the same site it is cool, but if you leave for another site, I prefer another window.
I also think it will be a good idea to warn users that a new window will open
From a website perspective I would prefer users to keep my page open when going to other links so that they can easily return to me. From a user's perspective I find it annoying if I go to a link and then have to re-find my original starting point - that being said, sometimes links lead to other links which can become really annoying as in the case of the free web-hosting service that we need to use for assignment 1.
Having published my portfolio just another word on this. I found that some websites can load quite slowly. It can really lose the flow of the primary website if they open in the same window. Also it is quite nice to be able to refer to each new website in its own space and it is an easy click to return to the primary one. I have just gone back to my files and placed the target _ blank tag into each of my outside link hrefs. I found it really annoying waiting for some of these to load.