I do understand why Nintendo and others don't do it but I do wish that on larger hardware like this the manufacturer of the hardware would include more than one miniSD card slot. It would at least reduce the cost of expanding to larger storage by allowing people to just add new cards when they wish.
Obviously, you can swap miniSD cards whenever you want but that's anywhere near as convenient as being able to just add a second or third card when you want to. It's not as if it's going to be an issue of space because it's not a pocket device and some pocket devices do include a second miniSD slot.
double SD card slots are rare as either you'll have to halve the bandwidth (which is already relatively low for SDcards) or put two separate disk controllers in the device. They're mostly only on high end cameras as a failsafe system for taking pictures.