Instant Runoff Elections

So I was wondering why in Atlanta, and many other regions of the US we have runoff elections if there is no majority winner in the primary election rather than simply doing an instant runoff.

Instead of voting for your most preferred candidate in the primary election, you would rank order the candidates. In this case, if there is not a majority winner in the primary election, you simply eliminate the candidate with the least votes and the votes are recounted. This process is repeated until a majority winner is found.

This process of voting only once instead of multiple runoffs saves the taxpayers significant expense of holding such elections. So I wondered what's the downside? Well there is plenty of debate. On the one hand, proponents of the instant runoff say that it not only is more efficient, but voters evaluate the primary candidates more thoroughly and increases voter turnout. Opponents on the other hand, say that although the primary candidates may be more thoroughly evaluated, the winner does not get as much scrutiny as they would if they had a separate runoff election.

http://www.sacbee.com/325/story/2371451.html

The following site talks about the benefits of the instant runoff in Australia:

http://www.fairvote.org/blog/2009/06/instant-runoff-voting-in-australia-guest-blog-from-ben-raue/