1000BaseT requires 4 pairs though (all 8 wires) whereas you could run two 100BaseT runs on a single Cat5 cable (great for adding a second drop without snaking a new wire through walls). 1000BaseTX runs on 2 pairs but requires Cat6 or better, but it doesn't matter because nothing you'd ever run into supports 1000BaseTX.
10gig on copper is likely different and I don't know enough about it to comment aside from how it's really uncommon in consumer-level gear right now. I believe a 4gig over Ethernet standard is coming soon which should be a good middle-ground.
