Open Source is Better - Ask a Virus Writer

Virus writers are not known for their morals - or for being fanboys: they will use whatever means necessary to achieve their dubious ends. So when MacAfee warns that

Malicious-software writers are increasingly using open-source methodologies when developing their code

you can be pretty sure that this follows an extremely objective evaluation of the various competing development methods.

Maybe not quite the testimonial most free software enthusiasts were looking for, but a testimonial nonetheless.