Boot to desktop and other options...
When Windows 95 was released you could still set your shell to Progman.exe (Microsoft even created a knowledge base article on how to do it)
Windows XP supported "classic" views for the Start Menu, Control Panel, Folders and windows
Windows Vista came with UAC dialogs to enhance security, which the user could turn off if they wanted
The theme is a common one for Microsoft. They introduce new UX features in a Windows release which are enabled by default, but they still support the older style via a settings option. In subsequent Windows releases they enhance the new UX features introduced in previous versions and work toward removing the legacy options.
The goal is to encourage users to use the new improved UX features, but not to force them to (at least until the next release)
It's not so much a question of whether the Windows 8 Start Screen is better than the old Start Menu. Personally I think someone would be crazy to want the old Start Menu back, but if there is an option for them to do that, and it doesn't affect me, I don't think there is a problem with Microsoft supporting it.
Start Is Back enables the kind of options that Microsoft could support (e.g. boot to desktop, enable "classic" start menu, disable hot corners etc)
As Microsoft continue to enhance the metro UI fewer people will enable legacy options, at which point Microsoft could move toward removing those options (which is going to happen much faster with yearly releases)
TL;DR Options are fine as long as they don't ruin the experience for everyone else.