Well, it's not a very good defense against patent trolls, because those guys typically don't exercise their own patents or anyone else's. However, it does help

Well, it's not a very good defense against patent trolls, because those guys typically don't exercise their own patents or anyone else's. However, it does help somewhat against companies like Apple or Nokia who are fine with litigating against anyone who tries to compete on an even footing. For example, if Yahoo or Microsoft sued Google for some search related patent, probably Google could use this patent against them.