I think proprietary has it's pros and cons. You can take a hard drive off a 360, plug it into a completely different 360 and your data and profile are there. Rock Band DLC, profiles, XBLA games...everything works and tranferring for an upgrade is as seamless as possible. The USB drives/memory cards are the same way.
I can do the same thing with an external hard drive. In fact I do it every day on multiple computers and don't have to pay outrageous prices.