When comparing food processor models, why do some have additional blades?

The maximum rating is based without the top blades inserted into the pitcher. This drives the greatest power.

TIP: When pureeing, use only the bottom blades for best results.

Top [middle] blades are needed to power through tough ingredients like ice, solid fruits and vegetables.

http://content.abt.com/documents/51012/BL830_OwnMan.pdf

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Bonus idea

For the prices I'm looking at ($100-200), it almost seems worth it to get rid of my current blender, buy a combination attachment blender--with food processor and portable blender containers:

Related: blades be sharpened or replaced?

The Robot Coupe is widely regarded as the standard food processor for professional use. Neither it, nor the industry-leading Cuisinart or KitchenAid food processors have those middle blades. As such, it seems unlikely that they are requirements for a quality unit.