The generator I used for a recent 2-week festival had a high and low setting, on a toggle switch. It was a 45kVA, and I don't remember the brand but it was a newer, reputable name-brand unit. The rental company was Sunbelt, probably out of Colorado.
Based on what I saw on the meters, I think the one I used wouldn't send any power out in low idle, but obviously it would be bad to switch it from high to low while energized.
It's a good thing the only damaged equipment for the OP was a trace on a crossover. That could have been a lot worse!
Well I used a more recent model from Star Rentals last month, and it also had a high and low speed switch on it. It was put to me that this switch shouldn't affect the voltage output, just reduce fuel consumption (like "eco-throttle" on the Honda EUs), but in my case, it was halving the voltage... not fun.
On my 45kW WhisperWatt, I give it about five minutes operating, no load, before firing up the main breaker. That's per the operating instructions provided by MQ Power. So, if people are warming it up in 'idle mode' instead of full load, I guess that would make sense to save fuel, but adding that extra step of needing to remember to adjust some throttle/choke/whatever to ensure proper operation- I'd rather just have to remember to turn on main breaker and that's it.
-Ray