After all its "just" maintaining a state and rules for possible transitions.
I've implemented my own web framework (just like every other Python developer ;-) and I've done my own finite-state-machines for workflows. Web Frameworks are large, error-prone and have a lot of nuanced details. Workflows done with FSMs are much more transparent and require far less work. Each time I've created an FSM, it's been a custom solution exactly matching the problem-domain, and taking less than an afternoon to implement (both coding the solution, and populating the state & transition tables). Thus I'd say a web framework is multiple orders of magnitude more complex than a custom rule engine.
-tkc -- http://mail.python.org/mailman/listinfo/python-list
