Most people don't keep their filing system very organized and when it comes to bookmarks even less so. Context is often a more efficient way of recalling what you did than a directory name - particularly over longer periods of time such as several months.
And of course, you can multitask across virtual machines - I often have two or three open at once because I need access to software which runs on a legacy version of Windows and I run Facebook in it's own exclusive VM.
At the same time I will be working on a project on the host OS. And there is no need for interconnection between any of them.
And VM's solve a lot of legacy issues, cross platform compatibility issues (e.g. windows phone apps) and Microsoft has already developed methods of integrating VM's with the host (see Windows Virtual PC and XP mode integration).