The intent behind HD watercooling is to remove fans, and any noise from the system.
I have quite noisy HDs, and if i'm to change it will be 15000rpm ultra-320. These drive *require* cooling even under normal conditions. And guess what they're far from silent.
Now sealing them with foam to prevent noise would just be asking for problems. Hence some kind of cold device designed to take the heat away from the surface of the heat spreader of the HD. And why not a WB ??
Now this WB hasn't to be as wide as the HD itself. Someone tried with a standard CPU block and it was allright. The main problem is mounting. With a HD-shaped block you can use other HDD screws or standard case screw holes.
A HDD has an operating temperature range, and usually it's better for its mechanics to stay cold - unless you've got some kind of gas powered HDD

NOT freezing cold, just room temp.