The idea is that it should in theory make it more realistic for conservative IRB delegates to vote for countries such as Japan perhaps, or USA or Italy - despite the fact that the sporting and tourist infrastructure in these countries should already demand consideration on their merits. Whether the decision will reduce the amount of dubious horse-trading among delegates, therefore, remains to be seen.
According to IRB chairman Bernard Lapasset, while it's important that the World Cup be held regularly in one of the IRB's "senior core markets," the tournament's commercial success means they can now consider placing the tournament in "new developing markets to assist the game's strategic growth."
How this all squares with the IRB's decision to award the 2011 World Cup to New Zealand is anyone's guess. I'm sure the IRB would argue that the Land of the Long White Cloud is one of their "senior core markets" but, in terms of revenue generating capability and infrastructure, New Zealand can be considered no more than a "developing market." After all, wasn't a core component of their bid that if they weren't awarded the tournament this time then they wouldn't be able to compete with unions with bigger financial clout in the future?