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Tomas Restrepo: First, I agree with Sam that this makes sense in a loose coupled world. Cool. Second, this can also provide some pretty painful debugging and head scratching (wondering why your argument's are being ignored?) Absolutely has, many times. However, I don't think that's entirely the way to go. I think published interfaces should be in essence immutable.
Clearly, .NET (or is it SOAP itself?) has cemented association between XML and interface is too strongly. Be back later today with something that may provide insight into other perspectives... some REST advocates may find this amusing too. For a hint... see Don Box's XML & Inclusion.