My point above is precisely that I see open-source pitched to companies as "look you can get the community working for you" but I think that's (a) missing the point of free software, and (b) probably not happening much. Dumping stuff on Github might get some pull requests generated, but you don't get a functioning community without investing heavily in it yourself. Companies that don't really invest in open source generally have ghost-town communities and aren't well equipped to harness the power of those pull requests to begin with - it goes both ways.
So really the debate here is not "do you hire an engineer full time or not?" it's "do you allow the output of that engineer to be open source or not?"
There's also a perk for the engineer: they work on stuff they can publicly point to as their portfolio full-time. Want to see me code? I don't necessarily have to have a bunch of side-projects on Github. Go check out the JIRA for these huge features I worked on!
2.) The developer has high status in the company and thus can do what he wish (up to the point). The company ends up tolerating it.
("Hey, we wanted to use MAAS to maintain our cluster, but it doesn't support our particular power controller, so let's send the patch upstream...")
For example, web hosts like GoDaddy do a fair amount of contribution to WordPress because they host a lot of WordPress sites and their customers will keep renewing their web hosting if WordPress is excellent and secure.