I guess the real question that one has to answer when deciding on a secondary bus is, what is its purpose? If the aircraft is an all electric aircraft designed for IFR IMC flight, the essential equipment becomes a longer list. There is no reason not to design an aux bus to address all of the avionics necessary to safely complete an IMC flight and approach if the primary source of power, the alternator, not the main battery, fails. Limiting the aircraft capability by purposely not powering equipment that would help to add to the safety of the flight, should not be considered an option when doing the analysis. Most avionics have on/off switches, so the option is always there to shed power if it becomes necessary.
Let's face it, your going to install a second battery that may have the same storage capacity as the primary, why not power the majority of the avionics and use that capacity. IMHO the purpose of the backup source is to get on the ground, not continue your cross country in IMC.
I guess all I am saying is don't blindly follow a recommendation until you have done your homework for your specific need. Perform a failure mode, result and corrective action analysis and then make an informed decision as to what you need. If you are a VFR only aircraft and pilot, then the need is totally different than an aircraft designed to be used for IFR/IMC use.