Well as for the conceptul concerns I cannot help, there are many reasons why anyone would want to dual boot and have more independent systems (flash vs card). But for many people there is of course no reason to use it at all.

As for the menu there is a way to set the default choice via cal-tool command and you can also set root device to 'ask' to show the menu everytimes.

https://maemo.org/maemowiki/HowTo_BootRootFSFromMMC