[kenjackson]

The message passing issue with HPC codes wasn't perf overhead of the messages. Rather it was that developing message passing applications was very complicated. But message passing apps, when written correctly gave very good performance.

With that said, you're right. If you don't care about performance, or if you have very large grains of computation then message passing is relatively easy (although so is just about any model with those requirements). The question is what about when you actually do care about performance and your grains aren't so large that doing communication half-way across the planet isn't acceptable? When I'm trying to get 60FPS in my physics engine, I probably don't want to use a web service interface.