1) Essentially no one knows about the author-izer system or understands how it works, especially outside academia. Readers in companies, poorer countries, etc., can't be expected to guess that the way to read an article is to go to the author's webpage and follow author-izer links (assuming they have been set up). What these potential readers will do is: search something on Google (or follow a link from somewhere else), hit the paywalled ACM DL page, give up. This convoluted system of "open-access from one place, closed-access from another" makes no sense.
2) For authors who actually post preprints of their work, yes, you can read it this way. But then you end up with multiple versions of the same work, that are often subtly different: does the author's preprint integrate reviewer feedback? does it fix some bugs that were found after the camera-ready version was submitted? And anyways, preprints posted on authors' websites usually disappear when they change institutions or retire, so it's not a good solution.
3) Yes, you can retain copyright on papers published with ACM, but then you need to give them an exclusive license to publish, so this still limits what you can do with your work (besides some narrowly worded exceptions). There is also a 3rd option of making the work open-access with no exclusive transfer, but this costs at least $700 per article, which is obviously excessive compared to the actual costs of hosting a 12-page PDF.
So I don't think it's fair to compare publishing with ACM and publishing on arXiv, because ACM is not open-access and publishing with them requires you to pay excessive fees or sign agreements restricting how you can publish your work, i.e., the opposite of what's in the interest of science.
1) Essentially no one knows about the author-izer system or understands how it works, especially outside academia. Readers in companies, poorer countries, etc., can't be expected to guess that the way to read an article is to go to the author's webpage and follow author-izer links (assuming they have been set up). What these potential readers will do is: search something on Google (or follow a link from somewhere else), hit the paywalled ACM DL page, give up. This convoluted system of "open-access from one place, closed-access from another" makes no sense.
2) For authors who actually post preprints of their work, yes, you can read it this way. But then you end up with multiple versions of the same work, that are often subtly different: does the author's preprint integrate reviewer feedback? does it fix some bugs that were found after the camera-ready version was submitted? And anyways, preprints posted on authors' websites usually disappear when they change institutions or retire, so it's not a good solution.
3) Yes, you can retain copyright on papers published with ACM, but then you need to give them an exclusive license to publish, so this still limits what you can do with your work (besides some narrowly worded exceptions). There is also a 3rd option of making the work open-access with no exclusive transfer, but this costs at least $700 per article, which is obviously excessive compared to the actual costs of hosting a 12-page PDF.
So I don't think it's fair to compare publishing with ACM and publishing on arXiv, because ACM is not open-access and publishing with them requires you to pay excessive fees or sign agreements restricting how you can publish your work, i.e., the opposite of what's in the interest of science.