No it will not hurt anything.
Quick lesson in “wire science” when wires that are laid next to each other transfer date/electricity they get “cross talk” actually it creates a electro-magnetic field. This will create errors in data transfer.
So they inserted a “dummy” wire that was just grounded out. This neutral wire would buffer out cross talk by absorbing the noise.
Here is an exerpt from www.HowStuffWorks.com
Ultra DMA increased the DMA transfer rate from ATA-2’s 16.67 MB/sec to 33.33 MB/sec. In addition to the existing cable that uses 40 pins and 40 conductors (wires), this version introduces a cable that has 80 conductors. The other 40 conductors are ground wires interspersed between the standard 40 conductors to improve signal quality. ATA-4 is also known as Ultra DMA, Ultra ATA and Ultra ATA/33.
ATA-5: The major update in ATA-5 is auto detection of which cable is used: the 40-conductor or 80-conductor version. Ultra DMA is increased to 66.67 MB/sec with the use of the 80-conductor cable. ATA-5 is also called Ultra ATA/66.
So in short: No it won’t hurt anything, if anything it could improve performance. But I doubt it.