Well Dynamic adresses can change, and normally this is OK for most things, but if your port forwarding then it immediately breaks all the good work you just did. While it will always try to give you the same address, it doesn't always acomplish the task. Also in the case of DHCP with uPnP, the routers uPnP can fill up. Think about a dhcp change then all the ports get auto forwarded, then it changes in a few days and the port tables fill up. This will lead to a router crash in most cases or an intermitant slowdown on the route as the memory slowly gets eaten up.