I would never want to use UPnP for the reasons that Manuel mentions. The design brief for Monowall states
m0n0wall is a project aimed at creating a complete, embedded firewall software package that, when used together with an embedded PC, provides all the important features of commercial firewall boxes (including ease of use) at a fraction of the price (free software).
UPnP is a feature of many domestic firewalls firewalls. If you feel that it is an important feature, you could argue a case for it to be included but disabled by default. However on the flip side, it is not a common feature on commercial firewalls. Beside this, I think that you would struggle to find someone in the development team who wants to write the code for it. This is after all an open source project and not run for commercial gain.