UPnP might be supported, but the vast majority of setups are still using router port mapping. Using random ports by default would seriously confuse users who already have trouble setting up their routers. I don't think these defaults should be changed. Sites like portforward.com already have a plethora of guides for various routers (using screenshots et al) that tell users how to configure their routers for mIRC. Changing these settings would mean all of these guides would have to be redone, which is taxing on the sites and would cause more confusion in the meantime.
