Livestreaming sites have put content fingerprinting technology in place and often respond very fast to takedown notices. Even the fastest of responses will still cause damage. So does that mean JTV puts in a system where content owners can just start taking things down at all like ebay does with auctions? I don't know, that's scary.
I would never cancel cable because I can watch sports on a livestream site. The quality is usually crap with foreign commentary. Odds are the people watching that content don't have cable anyway (like myself).
There are two missing pieces to the content side of things when it comes to shifting people away from cable subscriptions: local news and sports. Local news could be solved in a fairly straight forward manner. Sports is up to each league,etc. If I were the NBA, NFL, MLB,etc. I would start working with livestreaming sites to do test runs of pay per view content. Once most sports games are ubiquitously available on the internet in real time and for replay Comcast should be scared. If Comcast actually had their shit together, they would have purchased JTV/Ustream/Livestream when they were cheap.
Right now, the real value of live video isn't from illegal sports or content, its from real time interaction. If you've ever seen a celebrity or music artist interact with their fans on a live video site, you know the platform is something special. The numbers that Jonas Brothers/Ustream/Facebook pulled in were absolutely mind blowing.