The Microsoft recommendation regarding the order in which the Exchange 2013 server roles should be introduced into a forest is the reverse of Exchange 2010. Mailbox role first & it does make sense. Why?
In the Exchange 2010 world, the order of installation was CAS, Hub & Mailbox, whereas in 2013, it is Mailbox first & then CAS. Why is it different in 2013?
The CAS role in 2013 is a stateless server, which is used for proxying, redirection and authentication. All the configuration objects are in the mailbox role. The remote PowerShell endpoint in 2013 is the mailbox role. Hence, if you do install CAS server first (which doesn’t break anything), you won’t be able to administer or configure any options using the Shell or EAC (as both methods need a PowerShell endpoint to connect to).
Until a 2013 mailbox server comes online, the CAS server is pretty much useless. Hence, the recommended installation order in 2013 is the reverse of 2010 – Mailbox role first & then CAS