Good points about closed source, but there are several points to consider.
It is true that it harder to write bots and other hacks for closed source system, but it is not impossible. Edonkey bot is an example, Morpheus-X another one. And it is aways possible to hack P2P client. By closing source security you get is pseudo security, it is still possible to do all the same things that can be done with open source, except it takes a bit more time.
Another point is that Gnutella is open source and there is nothing majorly wrong with the system
colinmacc's argument is a good one too: nobody owns Gnutella and that means that RIAA has nobody to sue
Of course I understand that Kazaa won't open their source as it's harder to make money on open source product. Ad displaying would be first thing that gets removed in open source Kazaa
