One of the pretty cool things is that Rack'em includes a web server utility for running applications locally.
Also Rack'em is technically Rack compliant, so you could literally run a Rack'em application with a Ruby web server.. Yes, crazy, but the unification is great. Why re-create the wheel with more specs and standards when the Ruby community has already created one that works?
I started this back in 2011, so stackphp wasn't around then, but who knows stackphp might be way better.
Personally I feel like a lot of the major PHP frameworks are bloated and lose track of simplicity. Rack'em tries to stay small, but still take care of a huge shortcoming with PHP.
Also Rack'em is technically Rack compliant, so you could literally run a Rack'em application with a Ruby web server.. Yes, crazy, but the unification is great. Why re-create the wheel with more specs and standards when the Ruby community has already created one that works?