We could write books about patents but a basic understanding is enough to have an opinion.
What is a patent?
A patent is a set of exclusive rights granted by a state to an inventor or his assignee for a fixed period of time in exchange for a disclosure of an invention. Pasted from Wikipedia, see http://en.wikipedia.org/wiki/Patent.
Why should we limit patentability?
If you read http://www.vanderworp.org/My_Message_to_the_World you'll understand it: "Both copyright and patents disturb free market working per definition - the government gives you a monopoly - and should be limited to acceptable proportions to serve society at a maximum." It is about seeking a balance between stimulating R&D and locking useful information. Too much patents harm our economies as they do right now.
What is the main problem?
Without doubt triviality is the main problem of the system. By patenting obvious things - which is quiet easy - legal blocks are created and abused between competing companies. Many so called software patents are trivial.
Do patents protect European competition?
Myth 8: "The EU needs a strong patent system in order to be able to compete with the rest of the world, the United States in particular."
Reality: Patents have territorial effect: European patents are patents for Europe, and not just for Europeans. Americans use European patents against European competition. See http://www.scriptumlibre.org/10_myths.
If you realize that only a quarter of all European software patents are owned by European companies you'll understand that three quarters of all European software patents are non-European companies saying to European companies: "You may not produce this or that without our permission after payment."
