I don't mean to disparage other Ruby application servers, but this is why I use Passenger. It's not that I believe other Ruby application servers aren't good, or that their authors don't understand Unix as well as the guys at Phusion, but there's a real dedication to stability and predictability inside Phusion that I don't see elsewhere.

You can acheive the same results with other app servers, but you're going to have to do a lot of the heavy lifting yourself. I'm not ashamed to admit that I have a lot more confidence in Passenger's solution than I do my own.