An interesting counterpoint from Bruce Schneier: https://www.schneier.com/blog/archives/2008/01/my_open_wirel...
In Germany this defense wouldn't really help you much. You're (partially) responsible for the crimes that are committed over your unsecured network. It's called "Mitstörerhaftung".
That kind of changes the math a bit. I don't want a dozen people torrenting off my network, not because I'm afraid of getting in trouble, but because it degrades my ability to use my network.