only profitable loot is the first 2, because when you calculate your losses of the attack + to annex only two loots is profitable, and then there is the 8 or so hours to wait for the resources to be produced, i think its best just to loot once then go back and hit annex. im speakling about beginning of era ok.
For the beginning of era you are right. First circle provinces are not that profitable but still i advise you to take some of that "free" meal.
Afterwards looting is THE way to gather missed resources/gold