Science Magazine’s April 29th issue carried an article by Harvard and University of Texas researchers with the title above. NPEs (Nonpracticing entities) are now filing ten times as many patent infringement actions as they did ten years ago. According to the article, NPEs are firms that generate no products but amass patent portfolios for the sake of "enforcing" intellectual property "rights." This suing of cash-rich firms is shown in the article to have a negative impact on innovation activity at the targeted firms.
Because of AIA (America Invents Act), firms may now defend themselves against "junk" patents through filing Inter Partes Petitions with the US Patent Office. Unfortunately, this is expensive and most cases end in settlement which encourages further infringement filings by "trolls."
The Science magazine article suggests creation of a mechanism ("advance screening") to weed out low quality patent lawsuits. 
Expert witnesses need to be aware that their clients may be on either side of such a dispute. I suggest that decisions about accepting cases require knowledge of the parties and law firms involved through similar advanced screening by a prospective expert.