A bit ago we had a rather lengthly disscussion on aftermarket verses OEM.
I took the position that all filters are NOT created equal and that aftermarket filters are built to a competive price point (which has to sacrifice certain features over the oem) while oem filters were built to the designers specs without the regard if a addition of a magnet, additional filtration media, a better over all construction or many other features raised the price a bit.
I am glad to see that you noticed the magnet. The lack therof just adds additional proof that aftermarket filters are NOT always (but can be with proper testing, and building to oem specs, which would then make the aftermarket filter as expensive as the oem filter) the best bet.
And yes, you NEED the magnet.
David