For me, it would be data transfer of large items like movies (i.e. for heading on the trip).
That's the only thing I could think of too. It would be nice if the tablets could do that or at the very least have memory card slots. Especially when tablets have such small storage at the moment.
Once tablets have more than 64GB of storage, I dont see the point of USB at all. You can put a lot of movies on a device with that kind of storage, unless the devices start supporting 1080P. Eh, as I mentioned...storage limitations are the only reason for USB and/or memory card slots.