Except for when it's not. According to the linked article, their business model is that they'll help protect you from the patent trolling of larger companies. For example, if you pay them $X,000 a month and you get sued by Microsoft for patent infringement, RPX will try to counter-sue Microsoft for infringing one of their patents.

It's not a complete solution to patent trolls, but it seems like RPX could provide a very legitimate service to companies that want to focus on building their business instead of worrying about getting patent-trolled by a competitor. It can't save you from getting sued by a completely non-practicing entity like Intellectual Ventures, but nothing else can yet either.