When does writing history have utility? Only when it has objective lessons to be learned beyond what is already commonly understood. Instead, most of Zinn's histories are written with the express purpose of revealing a seedy/disgusting/greedy nature of humanity which is already very well-documented and really doesn't need any help in being established. Part of my problem when reading People's History was his constant asides to point out despicable immorality. Yeah, no shit, you don't need to make it the focal point if the clear, objective facts are already painting the picture for your reader.
The thing that separates good modern historians like John Lewis Gaddis (somewhat, though I've seen Gaddis slip a couple obvious judgments on occasion), Will Durant, or Leo Strauss from Howard Zinn (and I'll admit, Zinn is sensational in his commentary and therefore gains a lot more attention) is the usefulness in it being objective and hands off. Presenting facts, what is known without stopping to make moral judgments (something that the reader should be entitled to do on their own).
I'd like to quote Noam Chomsky because, after all, if someone else says it better, why not quote him.
"He's made an amazing contribution to American intellectual and moral culture," Noam Chomsky, the left-wing activist and MIT professor, said tonight. "He's changed the conscience of America in a highly constructive way. I really can't think of anyone I can compare him to in this respect.""
And this one by James Carroll:
"Howard had a genius for the shape of public morality and for articulating the great alternative vision of peace as more than a dream," said James Carroll a columnist for the Globe's opinion pages whose friendship with Dr. Zinn dates to when Carroll was a Catholic chaplain at BU. "But above all, he had a genius for the practical meaning of love. That is what drew legions of the young to him and what made the wide circle of his friends so constantly amazed and grateful.".
Not saying which is better, Id say theres some merit to being openly subjective rather than being "objective" as it would seem impossible to be objective unless you want to reduce people to nothing more than numbers.
However, I havent read his books so I cant comment on them.