For most apps and servers, migrating the IP state is not enough. The app server's connection state cannot be easily migrated. E.g. The MySQL connection of a failed MySQL server will be gone. Migrating the IP connection to a new MySQL server won't do anything.
Carp is good for firewall server redundancy because IP state is all that firewall is maintaining.
Comments
For most apps and servers, migrating the IP state is not enough. The app server's connection state cannot be easily migrated. E.g. The MySQL connection of a failed MySQL server will be gone. Migrating the IP connection to a new MySQL server won't do anything.
Carp is good for firewall server redundancy because IP state is all that firewall is maintaining.