You mean "enable them all over again for every new DHCP assignment, unless you insist on static IP assignments".
Different devices may need to use VoIP, P2P, games and other applications that cannot be strictly mapped to just one system or even just one port. UPnP handles dynamic mappings, so you don't have to update your port forwards every time.
UPnP might not be the right solution, but there is a clear and definite usability issue when it comes to networking and online gaming.