The options do not display the full features of the mode. That being said, deleting .debs is a good behaviour, especially on a mobile device.

Everytime you install a package, it pulls a .deb, installs it (plus all the dependencies involved), if the app manager didn't delete those .debs, it would mean that every single package uses twice the amount of space it says on the box. How many people would be confused by this? A lot.

Maybe you know, off-hand, that you need to clean the apt cache in /var/whatever, but most users don't understand this, they don't even know what /var/ is. We've seen enough topics about the filled-up rootfs, let's not make the problem worse.

Why do we not want the install .debs to be put on the MMC? Because the MMC can be pulled out at any time (microSD or USB cable being plugged in). I don't know the implications of such a use-case, but I can't imagine it working out slickly and smoothly.