Exactly how useful is an airborne chainsaw anyway? I mean the collision would hurt like hell, but I don't see it doing any more damage than an ordinary blade being propelled in the same way, seeing as it wouldn't exactly have the time to saw through much without anything to hold it in place.
Practicality isn't an issue, it's how badass you look.
And nothing is more badass than using chainsaws in inventive ways.