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Our project (the Solaris packaging system, IPS), relies on posix_spawn() primarily for the ability to fork without making a large memory reservation (and possibly failing) because the forking process was itself very large. That's the (a?) bug benefit of posix_spawn() -- it's not a benefit for the programmer using it (who might have to fall back to fork/exec), but for the end-user that benefits from its streamlined operation.
You're right that it doesn't handle everything that subprocess.Popen() does -- though at least on Solaris there's a way to change the cwd in the file actions, and I'm sure we'd consider adding a way to do the setsid() as well. The rest should be possible cross-platform.