For the last week I've been flip-flopping over the need for a CAS (Central Administration Site) in my environment. While my network isn't very large from a client standpoint (1200 Workstations and Servers in the US and Canada) we are trying to improve redundancy wherever possible in support of a growing company.
My original mentality behind running a CAS + 2 Primary Sites are as followed:
1) Split management traffic between our eastern and western datacenters and provide deployment points for our two DirectAccess Gateways to service external clients
2) Allow for an SMS Agent and a full copy of the DB in the west to make it a bit easier for me to manage (the eastern datacenter is our primary)
3) Allow for scalability if/when we expand overseas
4) In the RTM version, you cannot add a CAS to a Stand-Alone Primary Site
While doing some troubleshooting and research into multiple SQL Instances on a single box for both a CAS and Primary Site DB (for my lab environment as well as production), I came across a couple of blog posts that really changed my tune.
The first one has to do with the storage requirement for CAS. I found out that even though the CAS does not service clients, it still maintains a copy of every package in its own DataLib and FileLib. As I have some storage constraints to deal with, this alone was a deal breaker for me. You can read more about it here.
The second one is something that I should have known during the discovery period as part of my pre-planning. Since new info comes out all the time, I won't be too hard on myself. SCCM 2012 SP1 is currently in beta and allows the ability to change the SCCM hierarchy; basically I can add a CAS to the mix later on once SP1 has been released. You can read more about it here.
What this does to my design, is basically simplify it to the same level that I have with SCCM 2007 currently. A Stand-Alone Primary Site will be used along with Secondary Sites for the western datacenter and all our regional offices. Stand-alone DP's will be used for any field offices that need basic software/OS deployment.
Here is another post with views on CAS implementation in SCCM 2012. He brings up a lot of great points about including a CAS in your SCCM Hierarchy, including some that I brought up!