When designing a protocol or programming language the inclusion of extensibility is essentially an act of humility. One is minimally admitting that one's design is not complete and more generally one is admitting that one's design is not perfect. By providing for extensibility one is enabling others to improve and in many cases fix one's design.
Unfortunately it's easy to get extensibility design wrong. Typically such design errors result from assuming oneself or others to be perfect and in that assumption one fails to provide for sufficient extensibility. A rather subtle example of this problem recently came up in the Web Services – Business Process Execution Language Technical Committee (WS-BPEL TC).
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