I'd love to film while I'm biking with something like a helmet cam. Fun stuff, like my daughter riding along, doing wheelies, whatever, but also maybe for evidence as I commute into work.
See if you can borrow one to see if it's worthwhile. I've had similar desires, bought the stuff, then the interest quickly faded. Make sure you won't waste your money.
As far as evidence collection, well, that's a personal choice, but I wouldn't. I understand the desire to have video proof of when somebody wrongs you. However, if you're like a normal person you violate many laws throughout the day (and you're probably unaware of what many of those laws ever are). That evidence is also recorded...
My rules for life in the USA (maybe it's different elsewhere):
1) Don't invite "the man" into your life (Once they start sniffing around, even normal activities can be twisted to make you look like a terrorist)
2) Don't collect your own data that "the man" can use against you (Everyday errors can be taken out of context and twisted in a story that may be factually true, but totally wrong about a person)