Comment 24 for bug 230973

if UPnP fails, ports may still be correctly forwarded, and if they are not, then the state of the client would be the same as in "i want active mode but i'll configure my router myself" settings option. there are chances it may still work so it would be unwise to switch to passive mode if the user doesn't want to. (and furthermore, in this case it would be like going to passive mode for 2 seconds at startup to get back to active right afterwards.)

getting the external IP via UPnP is just an added bonus but not at all a goal of the feature; actually the (previous and current) behavior is to not consider it an error if we can't get the IP via UPnP, as long as the port redirection worked.
if no external IP could be fetched, it's no problem, the client will simply send 0.0.0.0 and let the hub set it.

as for having multiple implementations, i just followed eMule as an example and this is how they do it. if MiniUPnP can be found to be reliable enough with all routers out there, sure, we could stick to it, but i don't think it is and having a fallback to the standard Windows COM interface is better for now to not upset people who had it working before.