I spewed about Greece's role in NATO yesterday in reaction to positive tweets by Secretary General Rasmussen. Today, he is in Turkey, since it is also the 60th anniversary of Turkey joining the alliance. To be fair, Turkey has been a bit of trouble for the alliance as well, although Turkey does provide more added value than Greece.
Both countries have challenged NATO's democratic credentials over time with Turkey having more frequent and more recent military regimes. Turkey did invade Cyprus and continues to be the only country to recognize Turkish Cyrpus as anything more than occupied territory. Turkey's policies towards its Kurdish population has been counter-productive at best. The Greece-Turkey feud is hardly one-sided, as Turkey blocks NATO efforts to build relations with the EU in large part because of Greece's effort in the opposite direction.
Yet Turkey provides more added value than Greece to the alliance as a bridge to the Mideast and as a significant contributor to Afghanistan (albeit one with tight restrictions).
All allies have their pluses and their minuses. Some countries more obviously contribute (Denmark), others more obviously cost (Greece), with Turkey very visibly providing both costs and benefits.