pros: if you're an early adopter, it's nice to know how the service you're investing time in is doing financially. One of the biggest risks of choosing a new/unknown service, as a consumer, is that it will fail/close and you'll have to migrate somewhere else. I think the open company movement has other benefits as well. Behind just financials, but companies sharing their employee handbooks & policies are a great resource for entrepreneurs.
cons: probably some. At times, depending on your long term plans, it's definitely better to keep your cards close to your chest. This model seems to work best for bootstrapped companies, and less-so for companies that go the VC route.
As a side question. Is there a list of other projects like this? I mean simple and to the point services for development?