From the ‘Developer Preview‘ files:
Microsoft’s Internet Explorer 9 is now out for developers to try out and test — well kinda/sorta. You see the IE9 Test Drive Platform Preview isn’t really a browser is it?
IE9 as it is currently available lacks tabs. It lacks a back button and it lacks an address bar. In my view, it’s a crippled browser that does not represent the modern web browsing usage model at all. The idea for Microsoft is to show off new features without the confusion of a full fledged browser — though why tabs, address bar and tabs would do that is beyond my comprehension.
In my experience, it is tabs, the back button and the address bar that actually help developers to determine if something works. There are plenty of security and performance issues related to having multiple tabs open at the same time. By limiting the IE 9 test drive, Microsoft if providing a pristine environment that doesn’t reflect the real world.
While I understand the motivation for the limited IE9 test drive, when you compare it to how their rival Mozilla is delivering a developer preview, Microsoft’s approach makes even less sense to me.