In the process of replacing a panel, I'm trying to find a reason to install a second nav radio in an aircraft that will have two WAAS units and one nav radio. I can see needing one nav as a backup to a GPS outage, but I can't come up with a reason to need a second one without resorting to contrived or implausible situations. Given the increasingly robust GPS constellation, you could even make an argument for zero nav radios, although I wouldn't take it that far. The closest would be an approach like the LOC/DME into KASE, which has slightly lower minimums than the GPS approach, but the MAP for the GPS approach is much closer to the ALS, so I would probably still run that one anyways. I'm having a hard time definitively saying there are no others. It would be no fun to be surprised with an airport that I needed the second nav for and be unable to run the approach wanted. Does anyone know of one or have other thoughts? Incidentally the panel will be G500 TXi w/EIS, GTN 750, GTR 225 (digital com, but they also make a GTR 255 that also has a nav radio, hence the question). Will usually have an iPad also.