Colour accuracy isn't possible we're aware of that. I'm more dealing with subtle changes of layers, transparencies, gradients, that sort of thing. I did re-test some of the viewers and most of the issues seem to have been fixed, but not all. Flattening isn't an option, the resulting files could end up quite huge, even downsampled. Our software vendors do respond to requests quite well, but if we tell them "ABC open source PDF viewer is doing this, but every other viewer is fine" it's not going to go to the top of the support queue.
Even if the other viewers were all perfect, it doesn't solve the other reason we need to use Acrobat: most commercial plugins and extensions are written for Acrobat. I think that's a valid answer to the parent "Who needs it?" even if you don't like it.