Question on Exchange 2013 Configuration

I am looking for some feedback on whether this is the correct setup for Exchange 2013. Current setup with Exchange 2010 is ....

Site A

2 Cas Servers (in casarray)

2 mbox Server (in DAG to Site B)

Site B

2 Cas Servers (in casarray)

2 mbox Servers (in DAG to Site A)

Moving to Exchange 2013, this is what the plan is ...

3 Servers in each location

Site A

2 Servers with CAS and MBOX Role (with DAG to Site B)

1 Server with just CAS role for redundancy (not sure if this is needed?)

Site B

2 Servers with CAS and MBOX Role (with DAG to Site A)

1 Server with just CAS role for redundancy ( not sure if this is needed?)

Does this sounds like a good solution or is the 3rd CAS server not necessary?

Any feedback would be great.

Thanks,

Gavin