The website I am reviewing needs couple of external plugins (flash and java) to be installed. Also the site uses a strategy where every popups and most of the pages opens on a different browser window.
In terms of usability the user experience is certainly been harmed but technically is this a good practice to follow.
Are there any other usability flow other then user missing the navigation path and too many browser window running in the background affecting the bandwidth and performance.
Opening a new window/tab for every page definitely impacts usability. It's unusual behavior and forces the user to "clean up" after the website by closing all the extra tabs/windows. There is a convention of opening links to other sites in a new tab while all the links within the site stay in the same tab.
The other big issue is that popup windows may be blocked by a pop up blocker, which is very common these days.
This thread has a lot of great information: Is there ever a good reason to force opening a new browser window?
In general, your plugins should be opened in a new window. For the other pages, if you could improve the flow by reusing a single window, then you should. Things should really only open in a new window if you have a good reason for that specific page to open in a new window. Definitely do not make it a design choice for all your pages to open in new windows.
It's not so much a question of bandwidth and performance but rather the flow of the user. Opening pages in new browser windows can offer a very broken experience and users will become quickly frustrated at this. If this is all to frequent, you can expect to retain very few users.
If a separate page is required whilst also keeping the user on their current page, then I would recommend opening the new page in a new tab.
Is pushing the user to a new browser window or tab absolutely necessary?
My guess would be no. In my experience, the only time I've designed and been able to justify opening a new browser window, is when the user requests some quick How-to tips for the web application they are using. Given that the majority of this particular apps users viewed the app on a browser less then the typical screen resolution, I was able to justify opening the how-to's in a separate window, thus allowing the user to have this to the side of the app, not interrupting their work flow.
