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.
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