This statement is not true now and it was particularly not true given what we thought in the time when the decision to get involved in Vietnam was made.
Ideally, US involvement in Vietnam would have created a stable, democratic, and capitalist state in South Vietnam. This would have been extremely valuable in a number of ways. First, it would have been very good for the people of South Vietnam, who would not have been subjected to communist rule. Second, it could have been good for the US economically. A capitalist South Vietnam could have been a boost to the world economy just as South Korea and Japan were. Finally, this would have been a major boost to American prestige in the world and a blow to communism. This would have been good for the US and good for the world.
Of course, we incurred many costs. We lost tens of thousands of American lives and spent tremendous amounts of money. The arguments over the war helped to pull our country apart to some degree. However, the potential rewards if things had gone right would have been worth the cost just as the rewards of WWII were worth the much higher cost of that war.