Well, using it wouldn't even create savings as it will use more power disabling the head parking.

But I agree, wdidle is not supposed to be used to affect power usage (whether to increase or decrease it... extra power consumption is only a byproduct), it is designed to minimize the wear created by causing the head to park. The drive is only designed to withstand a certain amount of head parks, and the default timeout on the older green drives could cause several hundred head parks in a day, which can cause you to exceed the rated amount in a short amount of time. One site said they'd noticed 500-600 head parks every day, which would cause the drive to exceed the manufacturer's rating in under 2 years.

I had a drive with almost 200k head parks in about 2 years. I have since disabled head parking on that drive to minimize additional wear and relegated it to second string...