It will not be long before 10,000 RPM IDE drives are commonplace. The transfer of large files, often written sequentially on the hard drive, is particularly affected by the transfer rate. Performance bottlenecks result between the host and the hard drive. This way you can use the new drives on your current controller. The new cable increases data integrity and reduces crosstalk.
Crosstalk happens when signals leak in to nearby cables.
The extra 40 conductors are ground wires between the 40 data conductors. The Promise Ultra66 includes an 80-conductor cable. If you need to get a cable there are a few vendors currently selling them.