I was at a wonderful Hacktober event last year (thanks, Setlog and FOSS-AG!) and think some pretty good stuff came out of it. The incentive to submit low quality PRs became quite obvious quite quickly however.
My gut says that Hacktober probably spawns some productive contributions, but most of them would likely have been submitted anyway.
From my perspective last year as a first time participant in other people's open source projects, I don't think I would've made any of the contributions I did without Hacktoberfest.
Projects tagged with the Hacktoberfest tag tended to signal either projects that were both active and had low barriers of entry for newbies, or weird mechanical turk-esque spam. While the latter is unfortunate, the former isn't nearly as easy to find as it should be the other 11 months of the year.
My gut says that Hacktober probably spawns some productive contributions, but most of them would likely have been submitted anyway.