Having one login and one account across devices makes a service a lot better. Obviously most services operate that way, but it's hard to appreciate how important that is until you run into a service that doesn't.
One service that doesn't have one login across platforms is news service Bloomberg. Its iPhone, iPad and Android apps don't share data. The result is that I've configured the app locally for each platform (a bad user experience) and that Bloomberg doesn't learn about my across device usage nor can provide true personalization. A loss for both parties.