Superior to two-round 21. In our society, for many reasons, it is hard to get many voters to pay attention to election campaigns. IRV increases the likelihood that the ultimate decision will be made at the election with the greatest level of citizen participation.
Runoffs tend to have a lower voter turnout, though there are, of course, exceptions.
Imagine the turnout for a runoff for a more minor office such as state Treasurer. Traditional runoffs are also costly, both to the taxpayer who must pay for the duplicate election and to the candidates who must resume campaign fund-raising and prolong the stress on their families and business lives. The cost of ballot retabulation in the case of IRV is a tiny fraction of the cost of holding a new election. Traditional runoffs can also create doubts about candidates' sincerity as they change their message and repackage themselves based on their narrowed opposition, and the new perceived swing voters. IRV eliminates one of the biggest confusions for many voters that traditional runoffs don't solve.