Re: JailBreaking Apple T.V.
The main reason to do it would be so you can install a program called Kodi (which used to be called XBMC) which lets you stream digital media, and run add on programs that let you view online content such as live streaming channels that may not yet be available on Apple TV. Some are more legal than others.
(for example Kodi had NBC Extra long before Apple TV got it officially and it WAS legal, HOWEVER you might also find a stream of HBO which probably is NOT legal for you to watch)
The main con is that as Keith said only the 1st and 2nd generation AppleTVs can be jailbroken, and the folks that have them know this AND they charge you a pretty penny when you buy them 2nd hand. It's not uncommon for them to go for hundreds of dollars on ebay.