Neither your battery nor your SD cards are hooked up via USB. Ask the Touchbook people how well WiFi PSM works on their internal USB dongle (hint, not well). Powersaving is not (and probably never will be) as aggressive on a USB peripheral as on a directly connected, soldered-on peripheral, thus, internal USB peripherals == **** battery life.
As far as hooking it up via any other means, modular components cost money and use up space (for an extreme example of how this works, see the BUG). Would you be willing to sacrifice a memory card slot for your modular modem?