Claim: Uncle blocks, which are used to reward miners when duplicate block solutions are found, contribute to the security of the main chain.
Argument/Evidence: See original argument/evidence here which originally rejected this claim. This was, in fact, a false claim UNTIL the most recent Byzantium hard fork, during which uncle blocks were included in the difficulty algorithm. Thanks to @nicksdjohnson for pointing this out here. Source for this in geth can be found here.