You are losing half the bandwidth on each drive interface but the WD Caviar Black is a 7.2k mechanical drive and it can't saturate a 3Gb/s SATA 2 interface. Since the X58 supports 8 3Gb/s SATA 2 drives I don't think you're actually losing anything.
Edited to add clarifications for your comment
The main reason these drives have SATA 3 interfaces is because of marketing, 6Gb/sec is faster than 3Gb/sec and it makes for a nice differentiator for the drives for the guys putting the sales pitch together but it doesn't mean its actually particularly useful. There are some edge case advantages for some extreme workloads but almost all of those will be handled better by OS level caching which negates the benefits for these drives for the most part, as you will find if you dig out any benchmarks comparing their performance on 3Gb/sec interfaces vs 6Gb/sec ones. For sustained IO the spinning media of a 7200 rpm current gen Terabyte size hard drive will never exceed about 1Gb/sec. SSD's are quite a different beast and the faster SATA 3 interfaces can be significantly beneficial with them.
RAID 10 reads on a 4 drive RAID 10 pack will hit 4x the bandwidth of a single drive but the X58 board you list supports 8 SATA 2 channels. Since SATA connections are point to point that means it allows all drives to transfer data back to the controller at their maximum transfer rates concurrently. The X-58's controller might not be able to realize a full 4x improvement in transfer rate for the RAID pack over a single drive but if that is the case it will be because of the capabilities of the controller's RAID processing circuitry not the SATA interfaces. A dedicated hardware RAID card would certainly deliver but it would cost a bit more.