This is something I keep trying to tell people whenever micro-benchmarks appear showing that such-and-such web server handles >5k concurrent connections within a particular memory footprint, or that this-and-that message queue supports 8k enqueue/dequeue ops per second without hitting disk.

How many Rails apps out there can handle more than, say, 200-300 concurrent clients without either pegging the CPU or bringing the database to its knees?

Putting Passenger behind Nginx just removes a need to install + configure Apache for folks who are already use and like Nginx. It may also have some benefit on memory-constrained environments like entry-level VPS hosts.

That being said, I personally think that folks who spend days or weeks tweaking their deployment to run in <256MB of RAM to save a few bucks a month on hosting should probably reevaluate their priorities.