I think it's a fine idea precisely for the reason that (given enough developers behind it) it has the potential to speed up the arms race. If spammers have access to the source code, they will be able to develop their attacks more quickly; as a result, we'll see those attacks in the wild more quickly, and more people will have the potential to develop responses to those attacks. Given enough eyes (and that's always the problem, isn't it?), those responses will be generated more quickly, and as a whole I think it'll increase the rate of advancement of the field.

In particular, there are loads of academic papers on search engines whose results just never get integrated into any real system because the closed-source developers are too busy and most academics don't really care about seeing their work actually implemented. Having an avenue for academic research to filter over into the real world is always a good thing, at least in CS.