British Prime Minister David Cameron's comments in support of Turkey's EU membership bid offer a good opportunity to underline an important point: The question of whether or not Turkey belongs in the EU was already decided when the union offered Turkey membership.
Clearly, member states enjoy a wide range of political methods for dragging out or blocking the subsequent negotiation chapters. Slovenia, for instance, has used its veto power to block Croatia's membership negotiations over a border dispute that the two have since agreed to resolve by arbitration. It's also worth mentioning that Turkey has played a significant role in hobbling its own accession process, both by its Cyprus policy and its half-hearted approach to democratic and judicial reforms.
But the fundamental question of Turkey's "Europeanness" is no longer up for debate. And the cost for the EU in readjudicating that question will not only be measured by what it loses in terms of Turkey's potential strategic contributions to the union, but also by the hit the EU will take in terms of its credibility.