I'm bummed that you've began blanketly discouraging your companies from working at coworking spaces, instead of encouraging them to find the right coworking space (& community) based on their needs.
I also find it interesting that you'd qualify multiple startups as sharing an office as acceptable, but other shared arrangements as not. I've actually experienced the opposite effect, where having a more diverse (as in, not just startups and their own echo chamber) membership leading to the discovery of new insight and opportunities that wouldn't have happened if multiple industries weren't being given an opportunity to work side by side.
If you're talking about shared office spaces, then I agree.
A coworking space, however, shouldn't simply be renting cheap space. They should be working hard to foster a community of members to grow and contribute to that space, and share benefits of everyone's contributions. I have to imagine that this comes with a lot of similar benefits that getting involved with YC seems to provide. Shared space is just the battlefield for those interactions. I might be making unfair presumptions, but from an outsider's prospective, YC seems to provide a similar context/battlefield with its investments.
I'm the co-founder of a coworking community space in Philadelphia called Indy Hall. What I think is important about coworking spaces is that they provide an opportunity for people to get to know each other on a personal basis before the working relationship takes hold. This leads to stronger potential co-founder matches, and overall, a higher probability of people that need to find each other finding each other.
In terms of ownership, we work hard to encourage our members to take ownership of the space that they inhabit. All of the best things about working at Indy Hall are things that our members decided should be there, and we helped make possible.
Coworking simply isn't about space, though it's easy to mistake it as such. Coworking is about context, and an important context in terms of organic innovation and the spread of ideas.
Steven Johnson's recent book, "Where Good Ideas Come From", explores places - not necessarily spaces, but places - where throughout history innovation has happened. He cites places like salons in London and cafes in Paris, and the roles those places played in true renaissance periods. The common thread was that those places tended to be populated by people with half-baked ideas, and they provided a context where it was possible for those half baked ideas (Steven calls them "slow hunches") to bump into each other. It's that serendipity that is often mistaken for a "genius" or eureka moment. It's that serendipity that leads to the magic you think you can only hope for.
At a real coworking space (not just a shared office, of which there are many who are mislabeled as coworking), interactions are designed to facilitate those possibilites, and accelerate that serendipity.