I think that you are right about defects BUT here's what there website states:

Indestructible Composite Shaft: Pound for pound, our composite shaft is stronger than steel. It flexes on impact and won�t break, kink or corrode. That�s why we�re confident enough to guarantee it for life. If we could guarantee it for longer than a lifetime, we would.

Minn Kota Limited Lifetime Warranty on Composite Shaft
JOME warrants to the original retail purchaser only that the composite shaft of the purchaser�s Minn Kota trolling motor will be materially free from defects in materials and workmanship appearing within the original purchaser�s lifetime. JOME will provide a new composite shaft, free of charge, to replace any composite shaft found by JOME to be defective during the term of this warranty. Providing a new composite shaft shall be the sole and exclusive liability of JOME and the sole and exclusive remedy of the purchaser for breach of this warranty; and purchaser shall be responsible for installing, or for the cost of labor to install, any new composite shaft provided by JOME.

I think that you could argue that if they advertise it to flex and not flex on impact and will not break, then it would have to be a defect in the materials or workmanship. I would bet you could argue the fact up the chain until you get hold of someone willing to just send you one. The warranty only replaces the shaft and not the labor to install a new one. Let us know what you find out. If they don't do right then it is probably much cheaper to buy a new shaft then a new unit.