There are Electrical considerations with generators as well. I have restored and repaired several old generators as a hobby. (Ham Radio, CAP, and home mechanic DIY'er myself) I have found that generators that have been out of service for quite some time, or have just "been run" without exercising the generator itself under some sort of load seldom works, or if it does does not deliver nearly its specified output.
Many of the smaller ones rely upon residial magnetism to self excite themselves, and must be " field-flashed" with a 12 volt battery if they have been unused for a period of time before they will work.
For those of you who own small generators at home, a good way to exercise them is to plug an electrical radiant heater in to the unit justm ake sure the generator has enough output to run the heater you choose.
In the days when executions were routinely carried out via electric chairs, the prisons usually relied upon a large diesel generator. A few weeks before the scheduled date, the generator was run and exercised regularly to ensure proper output. Caterpillar listed in its generator manuals a testing procedure involving a 55 gallon barrel of water with a certain amount of salt added for conductivity, and a conversion chart for the time it took to boil the water with the outpit of the generator, versus the output of the generator.
The argument against regularly switching your center to Emergency back-up power has to do with modern electronics such as your 911 switch, PC computers, etc. If proper UPS is provided, or the switching is a "Soft switch" switching one of the 3 phases at a time, this should not be a problem. IT also tests the UPS system.
I agree that once monthly is NOT an adequate test. Also someone needs to do a "walk-around" visual inspection of the generator both while it is running (wear ear and eye protection) and afterward, looking for fuel or coolant leaks and other obvious problems so that if found they can be repaired before the unit is needed and does not work properly.
Regular service by a qualified company who services generators, not just engines guarantees a long life for your generator.