"This isn't theoretical, more than once while I was the program manager for XML Schema technologies in the .NET Framework I had to take conference calls with customers who'd been converted to the 'contract first' religion only to find out that toolkits simply couldn't handle a lot of the constructs they were putting in their schemas.Those conversations were never easy. "
My first reaction was to point out that maybe the toolkit itself might be an obstacle to building a large interoperable and interoperating system. Then I thought I should at least try to figure out what Dare might mean by 'interoperable'. Perhaps his concern is to build a low-effort adapter to existing software and existing toolkits. My idea of interoperable is to maximize the number of other parties your software interacts with and only secondarily minimizing the work to plug into that network.