ironically it's the very thing NRL are looking to outlaw given the risk of concussions, and whilst i agree it's good viewing i tend to agree about the risk of head contact to the defender, it's most common in NRL, but you often have the defenders sandwiching each other and it usually leaves one of the tacklers without somewhere to stick his head.
Unfortunately, that doesn't suit the rugby narrative, where we seem solely focused on protecting the ball carrier and have, thus far, neglected the risk posed to defenders.