Like other streaming services, Rdio had to make a decision about devices. Specifically, how the application should behave when more than one device attempted to use the service at the same time. If you run a service where a single user pays a subscription fee, then it’s in your best interest as a company to limit use to that one paying customer. If they’re sharing passwords, you’re losing money and taking a performance hit as well.
Other companies have dealt with the same issue, but they typically handle it in one of two ways: either they let the devices stream at the same time (with a maximum number of allowed devices), or they throw up an error notification that the service is in use elsewhere. The first option sucks for the company that has to eat costs and server load, and the other sucks for users who just want to access their content.
Rdio worked out a solution that I absolutely love. Instead of throwing up a message blocking the user, they framed it as a tool for using the app better. Each device that isn’t actually streaming music can be used as a remote to control the device where it is streaming. And the thing is, it’s super useful. I’m constantly using my phone to change music that’s playing on my computer. For me, it’s one of the most delightful things about Rdio as a music service. And it was all born out of a design constraint.