Well one reason to keep their own is for privacy of owners. But your points all seem reasonable.McLaren could certainly keep their own ownership ledger, although I don't really see why they would do it on a public blockchain. Immutability of the blockchain doesn't really confer much benefit from McLaren's perspective, nor does decentralization. Having a public ledger could help people understand how ofter cars change hands, but it could also reduce anonymity for customers, depending on how it was implemented. If someone can figure out the identity of a buyer from one car, they could then look for all the other cars that identity has owned on the chain.