The ATV2 can be jailbroken; however, to use the fun stuff, you'll want to check what the firmware version is, and whether you can downgrade if it's on the latest firmware. While there is a jailbreak available for iOS6.1 (aka 5.2 on the ATV), XBMC is not yet supported on this version of iOS. If these ATV2 models already come with the latest iOS on there, you'll likely need to wait for XBMC to be updated. (I do not know if the firmware can be downgraded; if it can, then you could go that route).
As for whether these are useful, I love my jailbroken ATV2. I use XBMC to stream WatchESPN and Amazon Prime movies, and I use Airplay to stream from my home server. Connecting a computer has its appeal (I considered doing so with a Mac Mini), but this interface is simple enough for my kid to use it, the footprint is low, and the price was nice.