Marvin you bring up a good point in terms of cost. A manual system can be done a lot cheaper than automatic.
However a manual system requires someone with the know how to be present to operate the system. The backup system will need to be started and stopped accordingly.
An automated system takes care of itself. Having a gasoline back up is really nice but again someone must ensure the gas is fresh. Someone will also need to refill gas cans during an extended the outage.
In many cases maintaining and operating a manual system is more of a deal breaker than money.