I'm also a single founder, but apply anyway. Are you averse to finding co-founders? I haven't found any yet, and I might or might not. I'm open to the possibility, and I believe YC may be a great place to make connections to finding a co-founder.
I've had co-workers who either don't add anything, or who actually reduce the amount of work or direction a project can progress.
I've also had (far fewer) working relationships where the you 'leap-frog' each other, pushing the entire project forward in leaps and bounds.
It's the second thing that I'm looking for in a co-founder. This is why I think being a part of yc or a fellowship might help. YC is apparently expert at picking great people, so it is a pre-filter of amazing people,who likely have a further network of amazing people. (not that my network isn't great, but most of my people don't share my interests).