It was recently on the tele that Thomsons do not automatically cover you for insurance in case of an accident while on one of their coach tours or activities whilst on holiday.
We have heard holiday reps saying dont book tours locally through Turkish people, as you wont be insured - but neither are you through your rep! This I think is just a ploy to get you to book their own tours.
The only insurance you can really depend on is the travel insurance you take out independantly. Also if you are planning to do sports such as paragliding, hot air ballooning or anything with an extra risk factor - jeep safaries may also come into this category - then its best to include it in your travel insurance in advance - often you have to pay a bit more fee, but worth it to be covered.
In the event of accidents - God forbid! you have to pay up front at Turkish hospitals, so we always take a credit card just in case.