I am thinking to minimize the number of ArcSoc as much as I can, so I decide to decrease the default value of minimum number of instances for each service to be zero not one, what is the Disadvantage of this decision?
As the best practice, you should keep the minimum no. of instances for "rarely used" services as 0. They could take slightly more time for initiating the instance for the first time, which should be a negligible difference. This will ensure unnecessary ArcSOC instances are not running, and could improve the Server performance.
It will help you to save some memory on the server, but as Jayanta said you can use it for rarely used services, otherwise it will cause performance issue since the user have to wait till the server creates the instance then the request will be processed!
If you are using a web mapping application such as geocortex or web app builder, the service should be invisible by default when the user loads the app.