That's a bit of a silly question to ask? WD Green drives are designed for low power, high efficiency systems. They are not performance orientated at all.
Although they are marketed as variable spin speed, they are actually 5400rpm. I've used several of the green drives (not in servers), and they achieve 80MB/s to 90MB/s which is very good for a 5400rpm drive.
I would not reccomend using these in servers. Watch the LOAD_CYCLE_COUNT in SMART. They really are not designed for 24x7 use.
WD Green disks are for low power, not performance. The Black Edition are for performance but of course with a power cost increase.
Also this editions are not designed servers but for desktops. Even when I do use Black editions for some servers (not production). The disks for servers are the WD series, at least for production servers. That said, of course you can use, Green, Blue, Black or any other disk on servers, but Green for example are low noise disks, in a data center noise is not a problem. If power is really a concern maybe they are the way to go if you want to be green, but a server disk is a disk that has to be 24/7 on. So keep that on mind when using desktop disk, desktops ram, etc. Example, desktop RAM is not ECC, that means the chances of the OS crashing is high, every 30 days, that means RAM goes into error, and you have to boot the machine, this is not a problem with desktops RAM because normally people reboot the machine at least once a day. The more sticks you have the more chances. If you have 4 GB ram non ECC chances of crashing because of memory is 1 once a week. Same can be applied in some way to disks, as they come out of factory to be used on a regular machine, not on a datacenter. Servers parts are always more expensive, but just because they are designed for a different type of use. But then again Google started with desktops servers, very cheap hardware.
The Green Edition doesn't save too much power when compare with the other Edition.
To save more power, we buy those 2.5" 7200rpm HDD. It uses less than 1W when idle and not more than 3W under load. The Green Edition one uses ~3-5W idle and 5-6W under load.
PYDOT, yes they are not design for 24/7 use. But, most customers will prefer a desktop grade hardware for server if this can save them monthly fee. For those that want reliability, they will pay more for SAS HDD, RAID and etc...