Such addresses are very useful, and it is strongly recommended that one such address be reserved for every cluster node so that the SysAdmin has access to a cluster node even when it isn't running any active services. For most systems, such an address per node is mandatory.
It is also recommended that one such address be reserved for every interface so that you can test interfaces and see if they work, even when they are not active.
Administrative addresses are required by the IPaddr resource agent for every interface, owing to limitations in the ifconfig command it is based on. The IPaddr2 resource agent does not have this limitation, but they are generally desirable for reasons mentioned above.