There could be different reasons, and they often depend on which vertical (e.g. Public/Government) you are located in.
This article aims to describe guidelines for the creation of service accounts that should be correctly configured to allow a SharePoint 2013 Farm to run using least-privilege administration.
The goal is to provide a per-Farm set of credentials, this approach will offer a good security isolation level. In most cases a service account failure will not make the complete Farm to go down but should make a single service to be unavailable. The worse a unique Farm can go down but certainly not other ones.
A suggestion is also to create Farms depending on their SLA level, indeed maintenance windows, support action or release management might need to be managed differently based on their criticality.
Click on following link to have a per-service account description of minimum required security.
Here is an overview of SharePoint 2010 application services that can consume SharePoint 2013 service. As a reminder SharePoint 2010 can consume a SharePoint 2013 service but not the contrary.
Service applications have been organized by SharePoint server version.
SharePoint Server 2010 includes customizable features and capabilities that span multiple product areas, such as business intelligence, forms, workflow, and content management. Customization introduces risks to the stability, maintenance, and security of the SharePoint Server 2010 environment.
Most of the customizations made by using SharePoint Designer affect only the site being customized. However, there are some customizations that can affect and potentially break sites and even impact a whole Farm. It is also possible to customize pages in a way that affect the maintenance and performance of the server running SharePoint 2010. This document intend to sensitize you with the risk related to an unmanageg usage of this powerful tool.
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