Funny you should say that. Back when Windows 95 came out, my computer sort of had to triple boot. It could boot Windows 95, for when I wanted to mess about with that. It could boot Windows 3.11, which my dad needed for his work (and since his work still used Windows 3.11, he didn't want to get used to a completely different UI), and it could boot into DOS for games (which was probably its most used boot option).
I say "sort of triple booted", because the difference between booting DOS or Win3.11 was really just a config.sys/autoexec.bat menu choice that did or did not start Windows. To start Windows 95, you had to hit F4 at exactly the right time (the same time that you'd use for F8 in safe mode). Kind of impressive considering this was basically dual booting from a single partition.
Of course, some games required such strict amounts of free conventional memory that we had to use boot floppies (or add more config.sys menu options; I remember having different options for with/without CD-ROM drivers and with/without QEMM, at least).
So not only did we have numerous different environments needed to run different kinds of apps, switching between them actually required a full reboot. Having to click the desktop tile in the start screen is really not that inconvenient by comparison.
No, it suggests that MS hopes to make most of its money from the Windows Store (which did not exist for Windows 7), for which they will need as large a user base as possible.