I agree with Barbarian about timing, this shouldn't be looked at as a mere quick fix to the RWC (ie bringing Gits in). But more to soften the blow of the ongoing exodus in future years.
1. We're losing a wealth of talent next year, we can retain some of that within the Wallabies now thanks to this rule.
2. It provides incentive for players to stay around for those extra few years to make themselves eligible.
3. It makes returning to Australia from playing overseas a much more attractive prospect. Beforehand, players would've had to return a season early, join a super rugby team and go through the hard yards. Essentially what JOC (James O'Connor) is doing now for example. This might not be appealing to a player like Douglas, whereas now he can slot straight into the Wallabies team (form permitting), giving him one more reason to come home instead of staying overseas.