There are two types of people in this camp, those that think web applications should be able to work in a browser-less environment (All toolbars, address bar, etc turned off) where the visitor has no access to the browser back button and therefore the application must compensate for this. Then there are those (like me) that feel the browser back button is already there and works fine and users know how to use it so let them use it. It seems to me that by building navigation so that the visitor can go back in the application is just re-inventing the wheel which is not really needed. Also research has shown that the browser back button is the most commonly used button on the web (around 30%).
Google provide users with a Previous button to allow users to navigate to search results on previous pages, I just wonder how many people actually use this Previous button. I must admit I would normally just hit the browser back button.
The main negative that springs to mind if you run an application in a browser-less environment then the application navigation needs to be absolutely perfect otherwise users will end up at a dead end with no way to get back.