It is wrong to prosecute the offenders in all cases. There are some very good examples of this
1. If someone is piggy backing on an unsecured system.
2. Material which is no longer available from any other source.
3. People who are too young to realise the illegality of it
Not everyone has the technological know-how to secure their wireless systems and in many cases, they are unaware of the system being insecure. As has been shown when Google did a quick scan of the neighbourhoods, the number of insecure wireless systems is huge. This gives downloaders easy access to people's high speed network connections. A user should not be held liable for the actions that they are either unaware is happening or do not have the ability to rectify.
On top of this is the "rooting" of a persons home computer. Rooting is a simple process by which someone else has access to another persons computer without their knowledge or consent. This has been seen with a number of viruses producing "bots". With this sort of access, the person who has rooted the machine is able to use the other persons machine as they like without the knowledge of the other person.
Forcing an ISP to provide potentially eronious information which leads to a prosecution where the user is guilty until proven innocent is also wrong – this can lead the ISP into legal trouble. How can an ISP know anything more than an IP address (the address of the computer the files are going to)? It is incredibly simple to fake addresses.
Some files have long been deleted and may be of significant interest for academic research or personal research (for example it is virtually impossible to obtain a copy of When the Wind Blows as broadcast on BBC Radio 4 as it has long since gone). In this case, downloading should be permitted (there is another argument that as it has already been paid for from the licence fee, it is not technically illegal)
Children are often the cause of illegal downloads and with the abundance of computers in bedrooms, it is hard to control and even if it could be, children are children and will do it anyway; it's very hard to avoid. Software can give some protection, but not a great deal. A parent should not be held to account for a child downloading (say Spongebob Squarepants) when they are too young to understand such concepts as illegal. A parent is not held to account if a child is under the age of criminal liability – the same should be applied here.