Turkey is not part of Europe either culturally or geographically.
Support for Turkish membership of the EU comes from those, such as the British government, who wish to impede European integration and workers' rights. With a populous poor member such a Turkey, setting meaningful minimum standards across the Union in employment, social security, etc.. would be impossible.
The admission of Turkey (or the Ukraine for that matter) would serve only to prevent the EU functioning as anything more than a trading area. I agree with those who say; Turkey should only be permitted to join after an EU-wide referendum, which would almost certainly return a 'no' vote.