@Ckepper asked about the potential benefits of PediaPress going open source with this project in this thread. I wanted to give them some good takeaways to bring back to their company as it relates to this specific project.
Commercialization of a given project is an important reason to want to keep it closed source. However, I believe it would impede the success of the renderer long term were it to remain closed source. As things stand, I do not think it will be as successful as Extension:Collection because, for starters, it would not be free to install for most. Small wikis would not be able to afford much in licensing. Open source also instills a kind of trust that any large company, nonprofit, or single individual can rely on. It shows you are so confident with your product that you will extend that to showing it for the world to see in its most basic form: code.
On a different note, as is reported on the company's website, "[you] offer consulting, customization, and support for advanced document transformation solutions." This is nothing small right here, and I am confident in that business model. If, however, you believe otherwise, there is still ways to protect copyright without going closed source. In this case, I would look to Chromium for guidance in what potential path you can take. Not every one of your ideas needs to be included in an open source repository, so you can still maintain the parts you want secret or just to yourselves.
The principle rendering service should, however, be available to the public to do bug tests and the like. It's a win-win. Consulting and customization are where the real money is anyways. You could also branch into hosting this rendering service for others similar to how you already offer print-on-demand books to any mediawiki-wiki. Wikis will always need to pay for this if they want the product beyond what is already offered out there.
Finally, it is a strong selling point for a company with such strong ties to the open-source movement! I hope this helps you make the right decision on this matter.