As you stated you may be able use the conversation state (outject result from the web service or something). You most certainly get some of the niceties that Seam brings to JSF (or you could use a different view layer) and if you end up having to generate emails,
RTF, PDF, or Excel files there's not much that's easier than what Seam does. Seam may very well make sense for the application, but it really depends on the requirements. If you're using JSF it's almost worth it just for what Seam brings to JSF. If conversations are possible (I find myself saying now, when would I not use them) or you need to do any of the above mentioned things, or even JMS and don't want to use MDBs Seam works very well.