I just want to start by saying this is a stupid argument to even be commenting on. We haven't got run off voting on the table and it never was on the table. The introduction of "Why not use Run-Off Voting instead of Instant Run-Off Voting (or AV)" Is an argument intended to obfuscate the debate and detract us from what we should be doing... extolling the virtues of AV over and above FPTP.
But, here is a post that explains why the argument is bunkum anyway. Don't engage with trolls for long periods of time, just give them this link!
In case you're adverse to reading long articles... IRV, or AV, would produce no different results in the UK, given our political makeup, than Non-Instant Run-Off Voting. To try and debate the fairness of IRV versus Run-Off Voting in the UK is to ignore that ours is a three party politics, and is a debate only introduced to try and wrong foot someone for kudos points that really aren't cashable anywhere outside of debate club.
The differences between AV and Run-Off Voting are small but important. AV asks individuals to give an honest preference list to be used to elect the highest supported candidate in an elimination style election. Run-Off Voting asks individuals to, round by round, state who they'd prefer from the remaining candidates while being able to alter their preferences based on who's left in the running.
It sounds similar, and in practice they are as good as identical, but there are some scenarios where not having an instant run-off is beneficial.
They are few and far between, and tend not to occur unless there are four highly supported candidates in the running for one post...but to clarify here is the situation under which AV is deficient...
How it works
Candidates A, B, C and D all have around 20% of the vote. Candidate A is most supported by first preferences, D least supported. The trouble (under both instant and non-instant run-off systems) is that D supporter's second opinions are now read before the supporters of A, B, C.
This happens regardless of system, D has no chance of winning. Under a non-instant system, however, D may look at the landscape different. Perhaps D supporters thought that they wouldn't be going out in the first round, and so their second preference was ill-chosen. Whereas under AV all of D's votes may have transferred to C, under a non-instant system those voters might choose to support B instead based on the numbers.
The real world
But in reality how is this really going to work? Most seats only have three main parties contesting them, if they even have more than two parties in the running. Those parties are the Lib Dems, Labour and Tories. Statistics from after the election show that Lib Dems would tend to give their votes almost equally between the two other parties, while the other two parties split almost equally between the lib dems and not voting for anyone else.
In seats where Lib Dems are the least popular of the last three remaining parties, they will most likely not change the status quo, unless the constituency itself is specifically more socialist or libertarian in it's blood. In seats where the Tories are the least popular of the last three remaining parties they'll give what votes they will to the Lib Dems, which may or may not push the Lib Dems above Labour. The same happens in reverse if Labour are the least popular of the three remaining parties.
Given this knowledge we have of people's opinions, are we ever going to be in a situation where the Lib Dem supporters being knocked out before the final round would have put their preferences down mostly for Labour under AV, but would have changed that to Tories under Run-Off Voting? No, we're not. Equally are Tories or Labour ever going to vote for each other because they have an opportunity before the next round of voting in a Run-Off election, as opposed to their original preference for Lib Dems under AV?
Those that are trying to obfuscate the advantages of AV by talking about Run-Off Voting are doing so without any sense of reality. AV is, at the end of the day, a way to put your true preferences on a piece of paper and let the collective opinions of the local area combine to find the best candidate in a knock out format. The idea that those true preferences would then change in such a polarised political society as ours is laughable.
There is a statistically fantastic thing that can happen whereby voters could be better off trying to vote tactically for someone else...for example Tories could recognise that by coming last their second preferences would transfer to Lib Dems instead of Lib Dems transfering to Labour, thus denying Labour a seat.
The likelihood of this scenario being present, however, is a) extremely unlikely and b) relies on voters knowing for sure what multiple levels of opinion will be in their constituency. Is that relevant in this argument?
It may well be an argument, a relevant one, for the condorcet supporters among us, but not for Run-Off voting, where exactly the same action is needed as under AV, with no additional opportunity to "rig" results due to the lack of diversity of supported candidates.
Again, just think... will a pause between voting rounds really make Tory voters vote for Labour as a second preference when they wouldn't have done so under AV?
So, aside from the use of the diversion tactics of the No camp to talk about systems other than the AV or FPTP on the table, this particular diversionary tactic doesn't even make any sense given how the results between instant and non-instant run off systems with our particular diversity of political opinion are almost indistinguishable!