Appears they don't "suggest" anything. Drives with intellipower vary in speed between sizes, but the drive doesnt vary it's speed.
Originally WD did in fact suggest a variable rotation speed and for some time refused to state a rotational speed.
This was their way to obfuscate consumer reception for the product, much like some CPU manufacturers dropped MHz ratings for 'performance ratings'. WD originally advertised this as a '7200 rpm "class" drive that uses variable rotational speeds', then after some guys measured the motor frequency during use to find that the rotational speed was fixed.
Further, using a variable rotational speed would cause heavy fluctuations in power demand and heat output, which would be contrary to the power savings objectives.
Here's an example of how the marketing-speak confused a leading expert at the time of release:
Seagate and Western Digital 1TB Drives: Improved and Green - AnandTech :: Your Source for Hardware Analysis and News
Tomshardware ran into the same "variable between 5400 and 7200" claim and republished it as well.
Here someone starts getting to the bottom of it:
Western Digital's Caviar Green hard drive - The Tech Report - Page 1
"When it first launched the GreenPower Caviar, WD refused to disclose the drive's actual spindle speed, saying only that it was somewhere between 5,400 and 7,200RPM. The company later admitted that the drive ran at closer to the former than the latter, but we haven't been able to coax out an exact spindle speed."
I'm a little pressed for time right now, but a later analysis measuring the frequency of the drive motors found that almost all of the WD 'intellipower' drives ran at 5400 rpm, with a single model running at 5900. While the statement that the motors could be set at the factory to run at any speed between 5400 and 7200 rpms is accurate, it was fairly meaningless once the drive left the factory and in the absence of other performance factors related to the drive.
The point remains that these drives run at a fixed rate of rotation which is similar to competitors drives, that originally WD sought to obfuscate the performance characteristics, and that it took over a year of cajoling and 3rd party testing before they backed off of implying a variable rotational speed.