Why would the voters listen to them if they did in fact approve of multiple candidates?
I do like approval voting as well, for the same mentioned reasons, but I worry that after a few elections people would start voting strategically, and then it could take another few decades to change the voting system again.
The more important part isn't ranked choice or approval voting anyway - both are single-winner voting systems which are far from ideal for elections involving Congress/Parliaments and local legislature. It's much preferable to have a proportional representation voting system for those.
What does “approve of multiple candidates” even mean?
Preferring one candidate over another has a clear meaning. Approval does not; as well as meaning honest, uninfluenced ballots are going to vary from voter to voter even with the same actual assessment of candidates, that makes it a lot easier for voters to be manipulated by influencers as to the right way to map preferences to ballot markings.
Approval is good for open-ballot voting on, e.g, where to go to lunch (where “approve” means “commit to attend if the option is chosen”.) It's a poor choice for public, secret-ballot elections.
If it doesn't, then your ballots have no consistent meaning and any process for aggregating them produces garbage output in terms of aggregating social preference.
Consistency of meaning of ballot markings and the resolution process being sensible in terms of that meaning are the most critical minimal standards for a voting system. Otherwise, voting is a farce and distraction, not a serious way of guaranteeing that government is accountable.
That isn't a reason to choose a system that maximizes the degree to which that problem is preserved.
It's still consistent in that in any case, the candidates you're approving of are all preferable (for you) to all of the candidates you're not.
There's no ambiguity.
I'll be an anecdote here, from 2016. I voted for Sanders in the Democratic primary. I voted for Johnson in the general election, but had I been in a battleground state where Clinton had a chance I'd have voted for her.
Did how she was doing in the pre-election polling and exit polls determine whether or not she was my favorite candidate? Of course not. Would my single vote for her to try to avoid electing Trump have looked any different from a devout partisan Democrat's vote for the same office if I had voted for her? No, not at all. What FPTP almost convinced me to do is to VOTE FOR SOMEONE WHO WAS NOT MY FIRST CHOICE.
That's the problem, you see. If I'm tempted to vote for someone I really didn't care for rather than who I prefer because of the mechanics of the counting, that dilutes my voting power. I wanted to vote for Johnson. If I could have ranked the general, it would have been Johnson, Clinton, Stein, Trump. If I could have voted Approve/Disapprove, it would have been Johnson and Clinton approve and the others disapprove. If it was Approve, Neutral, Disapprove for each I'd have either voted J+C, S, T or J, C+S, T.
The last thing I want is to have to choose between my candidate of preference and some tolerable major party candidate based upon who else is running.
Perhaps, but the question was why people seeking to replace FPTP advocate IRV instead of Approval, so pointing out FPTP has the same problem being pointed to in Approval when compared to IRV is missing the point.
The last few ranks in ranked choice voting go unused or are randomly filled, except that making an explicit ranking of a least favorite candidate requires ranking them all. The first few ranks also may well be random, as the first two or three candidates may all be equally acceptable to a voter. On a ballot with city council for a ward, council members at large, mayor, county board, county chair/county judge, school board, sheriff, constable, state House, state Senate, governor, lieutenant governor, comptroller, secretary of state, land office commissioner, agriculture commissioner, several judges, school board members, US House, US Senate, President, and more that's a lot of ranking and a lot of time to do so. It may no better support the voter's will than Approval after all the effort by the voter and effort to update the counting process. However, some people may well have a strong opinion on where each candidate in a seven-way race belongs in a list.
Approval/Disapproval or Net Approval voting, especially with a neutral option, solves that problem of having to support candidates you don't care about higher or lower relative to other candidates that neither RCV / IRV nor FPTP solves. In FPTP, you can choose just one and either Approval or RCV solves that. In Approval you can denote clearly that you approve of any number of candidates, without your first four choices being arbitrarily ranked. You can disapprove of any number as well, without your last choice of disapproval getting ranked right behind your choice for winning the office.
I genuinely can’t comprehend the problem you’re raising. Even if it can’t solve Arrows Impossibly Theorem, and even though I’m leaning towards technocracy rather than democracy anyway, it doesn’t seem worse than the current systems.
http://scorevoting.net/BayRegsFig.html
And this is true regardless of whether there is any objective "meaning" to the votes.
In mathematical terms, approving the candidate means you prefer that candidate to the expected utility of the winner.
This isn't a hypothetical. Queensland introduced "optional" preferential voting and one of the majors promptly campaigned on "Just Vote 1" in order to wedge the opposing coalition.
What Queensland uses, (which they call) Preferential voting, is IRV (instant-runoff voting). That's not approval voting.
It might initially seem like IRV's ranking (A>B>C>D>E) would be a better voting system than Approval Voting (where voters simply say A-yes, B-yes, others-no). Relative rankings seem at first glance to express more information. But it turns out that the way you have to count relative rankings either leads to pathological consequences for 3rd parties (see IRV), just like winner-takes-all voting, or the counting method becomes complex and byzantine in corner cases (see Condorcet). It's all explained on rangevoting.org
Most real and proposed balloting methods (other than range/score voting) are preferential, they differ in the restrictions on he expressed preferences; IRV (and lots of other systems), as usually defined, relies on a fully-ranked forced-preference (no ties) ballot. Approval has an implicitly fully-ranked two-preference-rank ballot with unlimited ties, FPTP (whether plurality or majority/runoff) has an implicitly fully-ranked two-preference-rank ballot with ties allowed only in the second rank.
> Relative rankings seem to express more information
They do, there is no “seem". They also do so more consistently. Forced preferences can have the reverse problem of forced ties, though (which is abstractly a point in favor of unforced ranking ballots, but there's less work done on resolution procedures for them.)
> but it turns out that the way you have to count relative rankings either leads to pathological consequences for 3rd parties (see IRV), just like winner-takes-all voting, or the counting method becomes complex and byzantine in corner cases (see Condorcet).
Bucklin is simpler to count than IRV and doesn't have it's pathological effects; the main concrete criticism I've seen is that if truncation is allowed rather than forced-preference, and if the no-majority fallback is highest-vote-total-on-last-round, it may encourage truncation and devolve to plurality. (But a similar charge of incentive to bullet-vote is made against approval.)