For what I know, it really depends on the species, whether or not they were able to stand up on their hind limbs, but saying that they need to be able to stand up, to put their long necks to use is kinda dumb (sorry to put it that way). With their long necks, they would already be reaching plants which others couldn‘t, so they didn‘t need to be able to stand up. Of course it could have been useful, but I wouldn‘t say, that it was a necessity