I would really like a more comprehensive answer from somebody close to the source; you more or less say "it does not have it because it has not happened yet".
Note that arXiv has features that many previous repositories did not have, such as easy access to the original TeX files.
Plus, a comment system for a repository of preprints sounds very useful to me -- you can have early birds (such as PhDs or very active scientists) comment on your paper even before you submit to a conference/journal, thus getting good advice for free.
Wikipedia told me this, but the homepage also advertises itself as an e-print service.
Anyway, "it does not have it because it has not happened yet" is exactly my point, because I don't see why it cannot evolve into a publishing platform.
Note that arXiv has features that many previous repositories did not have, such as easy access to the original TeX files.
Plus, a comment system for a repository of preprints sounds very useful to me -- you can have early birds (such as PhDs or very active scientists) comment on your paper even before you submit to a conference/journal, thus getting good advice for free.