most of the time replacing the unit complete is more cost effective than repairing the old one. The labor for the swap out is alot cheaper than the teardown, cleaning and reassembly and setup when rebuilding one.
Toyota knows this which is why they would rather sell you a complete part than a kit.
Also consider this, you get a parts warranty on a complete unit but do not on the kits unless you let the dealer rebuild it and that probably won't happen. The dealer usually does not condone the labor intensive rebuilding because of the labor cost to the customer unless the customer just insists it, they usually won't because of the high labor rate.
From my experience toyota likes selling parts, that's how they make up for the cheap selling price of the lifts. ;o)