I'm not sure how that makes it less portable.

External displays are decidedly non-portable. Why carry unused GPU hardware around when the display to be driven is back on the desk. This sort of presumes that displays at your destination, will also have their own GPU. But look at TV's, they have started embedding compute power already, enough to handle internet streams.

With Thunderbolt, there is a potential that computers will become very modular in a way not seen before.

Your desktop might be a brick containing a psu, cpu, memory, and a pair of t-bolt connectors. Everything else moves onto "the wire". Motherboards get very small.

Expansion or, need a Beowulf cluster, just daisy chain on a few more bricks.