The problem: People don't care about content and site quality. It's all about the links. Sites get spammed with crap content, sites get WAY too many OBL linking to the spammiest sites from all the guest bloggers.
Google is entirely aware of this. If you do it that way and don't care about your own site, WHO is actually blogging, how good/useful the content is, WHERE do all the links go...guest blogging won't get your site (and neither your blogging guests) very far.
IMHO, it's a good idea not even to announce it in public if your site accepts guest blogging and keep a smaller number of guest bloggers...common sense.
For Google, it doesn't make a difference whether a low quality site is some bot-site/scraper site, blog network or a "real" site but with tons of crap content pieces per day from real people. It doesn't matter.