It benefits both brands.When BMW and Toyota teamed up for the Supra Toyota took the engine away and retuned It and ran the bollix out of the internals.They have some rule where they only ever allow their engines to driven within 80% of their limits from the factory.They've a reputation to maintain.Their cars aren't very exiting so the reliability thing is important.
Their usually slower and less powerful than the competition for this reason.
I see PSA are bought Opel and are merging with Fiat.Hopefully they can improve their reputations too.