How is RCV better than traditional runoff and "top two" voting systems?
In a traditional runoff system, if no candidate receives a majority of the vote in the election, a separate runoff election is held at a later date between the top two vote-getters. In Louisiana and elsewhere, a traditional runoff system is used in conjunction with a non-partisan blanket primary, also known as a "jungle primary," in which all candidates from all parties compete in a single primary election. It is possible for two candidates from the same political party to advance from the jungle primary to the runoff. A variant on the jungle primary system is California's "top-two" system, in which the the two top vote-getters advance to the runoff even when a candidate receives an absolute majority in the primary.
Like ranked choice voting, traditional runoff systems ensure the winner has a majority of the vote. However, they attain that laudable goal in a crude fashion. Unlike RCV, which welcomes new voices and choices to our elections, traditional runoff systems yield majority winners by excluding candidates from the final, decisive election. Top-two systems are the worst offenders in this respect, because the final election is guaranteed to only have two voices in the debate. To make matters worse, those two voices may be from the same party, so the breadth and depth of the electoral debate may be especially limited. While traditional runoff systems are superior to plurality, for those interested in expanding the range of choice on the ballot and deepen our political debate, they fall short.
Traditional runoff systems do not eliminate as many spoilers as RCV does either. Consider a primary election between 6 candidates: 2 candidates from party A, each of whom get 20% of the vote, and 4 candidates from party B, each of whom get 15% of the vote. The two candidates from party A would advance to the final election, despite the fact that 60% of the voters chose a candidate from party B. RCV fixes this problem by simulating a series of runoffs, each of which eliminates a single candidate, but with only one trip to the polls.
With the exception of Cambridge, cities in Massachusetts use a top-two system for their municipal elections. If there are three or more candidates for Boston Mayor, for example, all three complete in "preliminary election" and the top two advance to the general. If our cities adopted RCV, they could eliminate these costly and time-consuming preliminary elections and find a majority winner with a single election.