The typical factory/aftermarket oil filter traps debris about 25 microns in size and larger. Yet many independent studies have found engine damage (wear) caused by ferrous particles as small as 2 to 5 microns. [Can we place a link here to one of the drop-downs? Specifically the GM paper, which I will have to send later] Removing this dangerous micro-debris is necessary for a cleaner running and more efficient engine, the key to longer engine life. Further, eliminating this debris, the primary factor for collateral damage, results in significant reduction of non-ferrous debris.

Still not sure about the value of complementary magnetic filtration?
Here is some expert opinion on the subject:

“In many applications, the practice of using [magnetic devices] for collecting and inspecting ferrous materials in lubricated systems is considered best practice and should not be disregarded” (our emphasis).
-From the November 3, 2004 Noria News-Tips Newsletter.
See http://www.lube-tips.com/focus/2004_11_03.htm for the complete quote.

“...particles in the 2 to 22 Micron size range are most likely to damage Engine parts . . . [and]
The smallest particles most popular filters captured with high efficiency are sized 25-40 Microns, depending upon the filter brand . . . [C]ontrolling the abrasive contaminants in the range of 2 to 22 Microns in the lube oil is necessary for controlling engine wear.”
-From Society of Automotive Engineers (SAE) paper 881825, written by David R. Staley of General Motors Corp.

Many references to the effect of “Engine wear” can be found on the Internet, in journals, and other scientific studies.