One possible reason might be the ability to integrate Seam into an existing Spring application. Existing spring apps may wish to take advantage of Conversation Scoped Persistence Contexts.
Spring provides some integrations that Seam does not such as Spring Batch.
Spring provides a more powerful TransactionManager (nested transactions)
Some people may just prefer the embedability and flexibility of JMS, various remoting schemes, etc. in Spring to that provided by JEE.
Why choose between Spring, Seam and JEE when you can use all of them together just fine? :) If you don't see a reason to use Spring then don't use it. If you like some things in Spring more than Seam then that should not stop you from using Seam.
Just a few thoughts.
Thanks, I am ramping up on Spring but naturally I was curious about any form of coexistence that may exist between it and some of the other technologies I have experience with. It's interesting that three work together fine but I would really like to avoid throwing another framework into the mix and introducing a framework hell sort of speak. We here are not making use of Spring 2.5 yet so are stuck in the XML configuration mess I hated with J2EE and so loved when I didn't have to have in JEE.