I don't think having access to source would help nintendo very much. They can't change anything in how the program runs, instead they can only change the *official* code that operates on their system, or change the hardware in future versions.
I'm sure nintendo has 'debug' versions of the wii they work with, and using one of them they can easily look at how and where each hardware component is being accessed and they'll know what is going on from that point enough to do as much as they can to counter act it.
On the other hand, open source is much better for the scene because if nintendo does find something to counter act it, hundreds and thousands of potential people trying to come up with another solution is alot better than, say, 3 or however many are working on the current project. Not to mention people looking at the code and coming up with other ideas, improvements, completely new projects (remember, this project surely has its roots in other homebrew stuff)...
