Turkey joining the EU would add 80 million more productive workers and citizens, and a very large economy to the EU, something it really needs. It would also be both an entrance and a message to the Muslim world on the EU's tolerance and growing strength, something it also needs to do.
Due to it's proximity to the neighboring Syria, I do not believe that Turkey is stable. The UK is advising against travel to the country due to the heightened chances of terrorist events. It seems Al Queda has an interest in targeting Turkey. Therefore at this time, Turkey is not a stable country and shouldn't be allowed entrance into the EU.