i think they are more or less useless, cause the Marian Reforms give you everything, what you need for a great empire, the praetorian units are only recruitable in rome and at the time you get the Augustan Reforms your empire is so big that the transport of the praetorian units would take you several seasons till you get them to the outer regions of your empire. in my opinion these slots should used for some other units, pherhaps some new units for KH, like special hoplites for athenai or something like that