It would seem that only a standby address that would be needed is for the "failover" interface. This is given with the following command for example:
failover interface ip failover_link 10.99.99.253 255.255.255.252 standby 10.99.99.254
Most config examples I look at have the outside int having a stand by address as well:
Interface ethernet0 Nameif outside Ip address 10.5.1.1 255.255.255.0 standby 10.5.1.2
Why would the outside interface need a standby address if when failover happens it would be using the IP from the primary ASA?