Getting back to the OP's questions.
We just traded in our 2012 S P85, with 95K miles, for a new X 100D.
We had the extended warranty on the P85 - and plan to purchase it for the 100D. While the battery pack and motor are covered under the manufacturer's warranty for 8 years and unlimited miles, there are enough other components to the car to justify purchasing the extended warranty.
The extended warranty is basically an insurance policy, protecting you from major expenses. Assuming Tesla has run the numbers, the cost of the extended warranty is likely around the average for repairs that will likely be done under the warranty (especially since Musk continues to claim they are not running service as a "for profit" business). This means some customers will save $$$ by not getting the warranty, while others will save $$$ by having it.
The extended warranty has a $200 deductible per issue though it also covers towing to the nearest service center (something that could be expensive if you need service during a road trip).
As for refreshes or other possible improvements to the S/X family - Tesla makes changes periodically - so you're almost guaranteed that no matter when you place an order, 6 to 12 months later, there will be something even better.
For us, after getting our S P85, we watched Tesla add many new features to the new Model S cars - power folding mirrors, parking sensors, TACC, AP, EAP, dual motor, … - we waited until there was a combination of features that made a big enough improvement before pulling the trigger on a new Tesla. For us - it was getting the longer range possibly with the combination of the dual motors and 100 battery packs.