The point of serving Node through Phusion Passenger is not performance, but supervision, stability, robustness, security, multitenancy, etc. Although Phusion Passenger can increase performance by load balancing requests between multiple nodes. Please see https://github.com/phusion/passenger/wiki/Node.js for reasons to use Phusion Passenger with Node.
The cluster module is great, but it requires you to manage your processes yourself and to write your own load balancer. Phusion Passenger provides all this functionality for you for free, through a C++ core.
