When you support multiple backends, you end up having to abstract over them. Each backend may implement the abstraction to the best of its capability, but you still have to deal with the abstraction sitting between your workload and its compute. Wouldn't it be nice if you didn't need that abstraction? That's what GP is talking about, I'm sure: optimizing the workload directly for the hardware, rather than merely the workload and the backend for the abstraction.