Re: if you got one of these letters, how do you defend the claim?
The first point, and most important, is that an IP address is not a personal identifier.
At most (and assuming the IP address was correctly recorded and time-stamped and correlated with your ISP's DHCP server) it tells them that someone using "your" connection (assuming you are the bill payer, most likely to get the letter) was attached to the bit torrent swarm.
Most countries do not make the bill payer responsible for other using the connection unless they have actual knowledge of misuse. A bit like you can't be prosecuted for your car being stolen and used in a bank robbery, but you can if you offered it knowing it would be used in such a crime.
These days most folk share there wifi password with anyone visiting their home, neighbours that have faults and need to get on-line, etc, as they have no reason to suspect criminal intent. In addition numerous routers have piss-poor security anyway and I believe Windows 10 is also going to make wifi sharing more common among acquaintances as well.
So basically unless you confess to it they have little case, the only real risk is they try and get a seizure of your PC to get it forensically analysed to prove the alleged copyright material was on a PC directly controlled by you. Possible, but a major cost to them and most of these trolling firms want a cheap settlement hence the threatening letters.
Given the time scale between the threat letter and any possible action, it would be unfortunate for the prosecution if your machine was replaced due to age, or had a virus and was securely wiped and re-installed before it came to seizure and court proceedings, as they could have a lot of expenses to pay to you...