They came out with a new standard so that the delivery bus is not the bottleneck. Though the drives don't support the speeds now, they looked ahead and opened the door for faster drives.
Look at it this way. Before SATA 3Gbps, the bus itself became a bottleneck because the drives hit the 1.5Gbps maximum transfer speed of the bus. So they designed a new bus. Now the hard drives are the bottleneck and need to catch up, but at least they have room to grow now.
Plus, just because you can't transfer at those speeds yet, you have so many new features available that weren't with PATA, hot plugging for one.