It couldn't hurt especially if you running RAID or SCSI, but a good fan blowing over them is just as good and alot cheaper. Besides adding the HDD block will add that much more heat to your WCing loop.
Eventually, I want to build a "block" for the drives (actually, much more drives). Unfortunately, I nearly went broke buying tools to do it and metal for failed experiments, so I cut my losses and just bought a bunch of Koolance coolers (aluminum, hence the transmission cooler).
I really wish there was more of a market for water cooling mass qty of drives, as then I could buy something pre-built and with just a pair of 1/2" ID fittings for the whole thing. That would be REALLY nice. Even something that cooled 3 drives in 2 5.25" bays would be much handier than individual blocks, and if they sold it for $100 (or less in qty), that would be really cool.