A smart print() implementation may check if there's enough output buffer, and, if so, quickly return a Future which has already completed. A smart scheduler can notice that and not switch to another green thread.
One can argue that in the VAST majority of instances, you'll never ever be printing so much that you'll fill the buffer. If you need that kind of control, just get a direct stream to stdout, otherwise make print() block if it needs to.
You might not fill the buffer. But your program might crash before the buffer is flushed. In that case having prints explicitly block until the IO is completed is very valuable, especially when debugging. Nobody wants to waste time debugging their debug code.
