One part you have some merit in your opinion, but that does not change the fact that most cities in America do not have enough ambulances in reserve. A private ambulance company is in it to make a profit and very few keep "extra" ambulances.
Police Officers have take home cars throughout America. One of the major reasons is so they can be called in when necessary for something like a riot or hostage situation.
As part of a requirement to get a contract with a city or county, politicians simply need to have essentially a take home provision as part of the contract. The only difference should be that the take home ambulances can be older models with the newest models in use all the time. Police are so thin at certain times of day because they can say they have many officers that can be called in as quickly as it takes for them to change clothes and/or hop in their squad car. It is not too much to expect something similar with ambulances?