Wow, Robert, I was grumpy twelve years ago, huh ?
While newer SLC-5/05 controllers support messaging to ControlLogix and to some generic CIP devices using EtherNet/IP protocol, they cannot create a cyclic I/O connection.
And that's how those AMCI devices are designed to work, with a cyclic I/O connection to a ControlLogix family controller.
For example, you'll see the Generic EtherNet/IP Module object parameters on page 24 of the SSI Module user manual.
While it's possible to send Explicit Messages to the Assembly Object of those AMCI modules from an SLC-5/05, there's no guarantee they'll work correctly if there is no cyclic I/O connection established. AMCI might support that, and they might not.
With motion control devices especially, the 16-bit operating system of the SLC-5/05 also rapidly becomes an issue with the ability to do math on large numbers of position counts.