This is a false dichotomy; taking advantage of language-level parallelism doesn't prevent you from supporting a full-blown distributed architecture. In fact, i

This is a false dichotomy; taking advantage of language-level parallelism doesn't prevent you from supporting a full-blown distributed architecture. In fact, if so inclined, you can model your distributed architecture on top of language-level (or OS-level) parallelism; message passing can be considerably cheaper when you can rely on simple shared memory mechanisms. Parallelism in the language environment allows you to make better use of the hardware you have. In a server environment, there's a fixed power utilization cost for each machine, and a per-CPU core power cost. It's cheaper and more efficient to scale up cores than it is to scale horizontally across machines. In fact, the more efficient you are, the longer you can put off the complexity of large-scale horizontal scaling. In a mobile environment, the equation is different -- you can't just add more mobile devices to scale horizontally. The device has X amount of cores, period. If you don't use them, your application's performance will not be up to par with other applications.