To me, it seems obvious that patents in my industry — software — are destructive. The Economist agrees, and makes some suggestions, in Time to fix patents.
Patents are supposed to spread knowledge, by obliging holders to lay out their innovation for all to see; they often fail, because patent-lawyers are masters of obfuscation. Instead, the system has created a parasitic ecology of trolls and defensive patent-holders, who aim to block innovation, or at least to stand in its way unless they can grab a share of the spoils. An early study found that newcomers to the semiconductor business had to buy licences from incumbents for as much as $200m. Patents should spur bursts of innovation; instead, they are used to lock in incumbents’ advantages.