A while back I ran into Marty Wasznicky and Scott Zimmerman excellent MSDN Article 8 Tips And Tricks For Better BizTalk Programming which is really worth a bookmarking. I really never got into their #1 Always Use Multi-Part Message Types (though the others I had picked up most of the hard way). I normally work with more messaging centric solutions and as a result do less with Orchestration, but the last year and a half has had me doing a lot more Orchestration and now I really understand this tip a lot better.

Although most Messages used in BizTalk Orchestrations are not multipart, using them in an Orchestration provides the benefit of isolating schema changes from impacting Orchestrations. Changing a schema without using multipart messages will require disconnecting all the Port, Send, and Receive shapes within the Orchestration Designer. This is a tedious an error prone process and it is not always easy to track down all of these affected shapes and there is no automated way to do it ahead of time. As the authors correctly point out; binding Ports and Messages within an Orchestration directly to a schema can lead to problems down the road. Usually these problems appear the first time you try to change something, which is often before even getting to production.

Granted there is one extra step here, defining the new part of the multipart message, but this is a small amount of work to do for a large amount of benefit. If you've ever worked with Web Services in an Orchestration (as a consumer) you've seen the Web Message Types that are generated for you; this is basically the same thing as using the multipart message types.