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USB has the massive advantage of backwards compatibility.
Thunderbolt has a lot of potential for as you stated, hardware expansion. I can see it being useful on low powered budget machines, to sell video cards in a box essentailly, that you plug in, then plug into the TV and get HD abilities or proper gaming abilities. Its also a big step towards what my ultimate want is, the modular PC. Pieces that you attached and detach as you need them, need more CPU, hook up an extra one, more video, and an extra one.