SecurtiyAgent process offload work to SecurityAgentHelper
Stability and Privilege Separation
XPC services are used to isolate sections of a process that require specific security considerations. SecurityAgent will pass SecurityAgentHelper work that needs to be isolated. It will not act as a fallback; this is not the intention of XPC services.
See Apple's Creating XPC Services for a detailed explanation of XPC Service's role and design:

The XPC Services API, part of libSystem, provides a lightweight mechanism for basic interprocess communication integrated with Grand Central Dispatch (GCD) and launchd. The XPC Services API allows you to create lightweight helper tools, called XPC services, that perform work on behalf of your application.
There are two main reasons to use XPC services: privilege separation and stability.