I think it would great to have dynamic user-defined rules. It is useful for API manipulation, especial for external connection tracking.
Complex QoS with Asterisk, which can set DCSP mark on their packets, but then traffic came from the outside perimeter of our network we haven't any DSCP mark, and we must use Layer 7 rules to determine traffic types. But, regexp rules not enough flexible for that situation, especial for IPv6 (we haven't any L7 rules). But, using API, Asterisk can tell the router to set a specific mark on their traffic. It can be a temporary rule, which automatically deleted after a timeout.