Manageability
The reliability of the AS/400 reduces the number of administrators required to support Domino. Mix this with protection and partitioning, and you have a much more manageable Domino environment. It should also be pointed out that Domino for AS/400 has an internal "watchdog" routine; if a Domino server fails, it's automatically restarted without disrupting Domino servers running in other partitions.

Scalability
The AS/400 has been designed to provide consistent response times to a high number of concurrent users, even if those users have a mixed workload (mail and applications). It may not always be as fast as a Wintel-based Domino server in terms of response time, but the response time will remain consistent as more users and applications are added to the server environment.

IFS (Integrated File System)
The AS/400 has a single directory structure that manages files across its storage system. This eliminates the need for drive partitioning. Adding storage to the AS/400 results in more space available across the single directory structure, meaning that you do not need to know where files are located in order to access them. This results in a more manageable and easily structured and supported environment.

DB2 and data integration
DB2 is IBM's relational database system. It's native on the AS/400 and (of course) can be directly integrated with Domino-based applications. The direct integration into the AS/400 environment not only provides increased stability, but increased performance as compared to attempting to do database integration between several different boxes running SQL Server, Oracle, etc.

Integrated messaging
Domino can be integrated into the AS/400 AnyMail environment. AnyMail gives sites still utilizing green screens the ability to send and receive mail to Domino and the Internet, thus providing a fully integrated messaging solution to sites with mixed client platforms.

Single source/directory synchronization
Domino for AS/400 may be configured to support directory synchronization between AS/400 user profiles and the Domino environment.

Why not the AS/400?
There are some significant disadvantages to the AS/400 environment. The biggest disadvantage to the AS/400 is the learning curve associated with the system itself. Fortunately, Domino administrators will find they rarely need to deal with the AS/400 environment directly, especially under R5. Still, it is recommended that you find a good Lotus Business Partner to work with (if nothing else as a backstop for support) and then make sure that the proper knowledge transfer takes place between your internal administration staff and the Partner's consultants.

There isn't as much expertise available for the AS/400 Domino platform as there is for NT, so it may be more difficult to find support. Fortunately, one of Domino's greatest assets is multi-platform support. Domino for AS/400 is not much different than Domino for any other platform.

It may be true that the AS/400 simply doesn't integrate well into your environment. If you have established database engines and development environment on other platforms, why switch if those platforms are already performing at a high level, and you have the in-house expertise to support them?