So the auto stop/start feature effectively only works when the engine is up to temperature and starting it is much easier, causing less wear on the starter motor.
My understanding is that the starter motor has been redesigned and tested to cope with the increased stresses of the additional starts, and the battery similarly redesigned to cope. I seem to recall reading that the starter motor on my vehicle is expected to cope with 150,000 starts?
In addition, before the engine stops at say traffic lights when you are braked, a number of conditions have to be met - I recount the following from memory, there may well be more
Not necessarily ruin it, but it will wear it out pretty quickly.
When your engine is used normally you might start it once a day, perhaps twice a day, if you are busy ten times a day. Now just one trip through London with the auto stop/start enabled will see the starter used somewhere around 150 times!
The thinking behind it is that it saves fuel and emissions that might otherwise be generated while the vehicle is idling away at a set of traffic lights. And it's rare, even when conditions are right, that a stop-start system will keep your vehicle's engine switched off for more than 90 seconds.