Jurgen's posted a great real world example of why WCF extensibility is essential for interop. The scenario he describes is that of having to a connect a TANDEM system written in COBOL to a Windows client using WCF. Which is an example of why you would need WCF extensibility even if SOAP is used.
But what if SOAP isn't used? Looking around I found Clemen Vasters's great series of blogs on exactly this topic. While it resembles more of an article than a blog, it's great because it shows as Clemens puts it, "...the fact that Indigo is using an information model that is aligned with SOAP means nothing for what goes on or comes from the wire..."