That digit span test is incredibly irritating. I've done it 10 times by now and I can't get more than 9 digits.
On a more philosophical note, I've wondered if there are some outstanding math or even societal problems that could've been solved already if humans had the capability of holding onto 13 or 15 more things at a time rather than just 7ish. It always feels like we're going to be limited in some way with regards to that if the problem is incapable of being broken down or collapsed.