I think the first software patents were for things like UI elements - the Lotus 1-2-3 design, or one-click ordering. I don't see any point in releasing code for something like that; the code is competely orthogonal to the design. You don't even need source to prove you are in posession of the invention; a working instantiation would seem quite sufficient.

Code only becomes relevant if the "invention" amounts to an algorithm. But algorithms aren't eligible for patents are they? Well, they aren't here. So ironically, if you need to provide code to support your patent application, then you've proved you aren't eligible for a patent.