Its hard to know what to make of these studies that show how a substance that is harmful/poison might be good in the long run. Like wine and heart disease: Should we all really add a glass of red wine to our daily diet?
Few substances are innately and incontrovertibly harmful or helpful. Even water can be deadly if you drink enough, to say nothing of what happens if you just hold your face in it for a few minutes — are you surprised by studies showing that 2 liters of water a day are good for you too? Iron is very poisonous too, but it's widely known as an essential nutrient. Ditto for some vitamins.
In the case of caffeine I suspect the main benefit is that drinkers of tea and coffee eat less food, and rich food damages the blood vessels of the brain (as it does to the heart).