When I specify in my preseeding file:
   d-i grub-installer/bootdev string default
grub-installer doesn't provide a query to install grub at all, and
rewrites mbr with no question, even an other OS is already installed on
disk. Installer also ignores "grub-installer/with_other_os" and
"grub-installer/only_debian" variables, when they are set.

Steps to reproduce:
- Run installer from regular Debian installation media on a non-uefi system.
- Specify 'grub-installer/bootdev=default' preseed variable as kernel
paramenter for installer.
- Proceed Debian installation, grub will be written to mbr without a query.