If we want the euro to be strong, there is nothing for it but to comply strictly with the convergence criteria. A weak euro and the attending high interest rate level would be of no benefit to anybody, and that includes the potential derogation countries. Please keep in mind that a monetary union with ill-converging Member States would be incapable of absorbing economic shocks.
Ultimately the tensions attending a malfunctioning EMU would jeopardise the achievements of the Single Market. In my view, potential derogation countries should not even wish to take part in an EMU which allows insufficiently converged Member States to join. That brings me to the position of Member States with a derogation.