Not sure what manual switch you are thinking of, but a properly installed and used one will only connect the house to the generator or to utility but not both at once. The only thing manual about it is that you have to throw the switch rather than pay a lot more for one that will throw it for you.
There are several ways to connect a manual switch. If not used properly, it creates a potentially deadly situation for others. It's not worth taking the chance that
someone may not perform that one important step! Automatic switches are cheap, and in my county (maybe even state) they are mandatory.
