When we need CAS in SCCM1012

I was working on  SCCM2012  design for a small environment and wanted to figure out if I need to incorporate CAS ( Central Administration Site ) in the design. Initially I was assuming that CAS will provide HA in the environment, but I was wrong . CAS is only required when we want to manage 100K+ clients  and when we have multiple primary sites. For a relatively small environment,  people are recommending to go with stand alone primary site.

Here are some links for a good reference on CAS: