I'm not sure about ALWAYS, i do not think its true. It is very hard to war when you play Tradition and you should take risk into accounts. Average Risk-adjusted NPV of war for tradition civ is definitely negative.Actually I think people are missing the forest for the trees.
The best strategy for a player has ALWAYS been early and opportunistic war. Even tradition civs are recommended to pillage and harass (If not always puppet or annex) neighbors.
However what absolutely true is that WINNING WAR SHOULD BE BENEFICIAL. By that i mean that if you decide to go war - you take risk. You have to build units INSTEAD of buildings. And if you loose the war - you are behind. Thus if you win - you should be ahead. Otherwise there is no need to go war at all.