Using the Management interface is almost like having management out of band as no other traffic is allowed to flow through it. So even in times of network congestion on interfaces on the ASA, your management interface will be clear for you to access (as long as you have a separate management network ;-))
Cons are you lose an interface that could be used for something else which you can do with the interface level command :-
no management-only.
Also the mgmt interface is only 10/100.
One more thing, stateful failover for the smaller devices tends to be configured on these interfaces as well.
