Creative control. I bugged Ronald for a year before he agreed to let me help on a trial basis. I'm sure I took the program in some directions he had not planned on, but on the whole it worked, but Ronald and I were the exception to a working team. However, I'll be honest, I don't see us adding a 3rd member at any point soon. I can very much understand Ski's wanting to maintain control and ownership of his source code.
Yes. Plus I know all the code right now like the back of my hand. If I let other people add code I lose that. I won't have the time or energy to look at an understand another persons code. I have to do that as as part of my career, and I tell you it is one of the least favorite things a software engineer does. Ask almost any software engineer if they like to fix bugs in other peoples code and you will probably get a #%$ no. So if I have to start doing that, it stops being fun to work on it. If it stops being fun to work on, it stops getting worked on.
And some of it is not written very....err...clearly.

Some parts are quite touchy. One little change in the wrong place could seriously break stuff, and many time the broken stuff is unrelated to what you changed.
So it isn't that I have not seen people that are more than capable and willing to help out, I'm just afraid I will lose interest once the issues that arise from working together on a project start to pop up.
So right now I am more that happy to let these talented people help me out in ways that don't really affect the main gameplay. Such as the Luck class, the proxy server, the replayer, the handling of key events in jre 1.4, ect.
One day I may rewrite the whole shootin' match. If I do that then I would solicit help. Christer and I have already talked about a good way to do it that would allow people to add plugins and replace certain elements. In that scheme if someone wanted to write a 3d field to replace the default field it would be possible.
P.S. Toby, there is a 4.8b out now that uses "1" and "0" in the apothecary fields.