Why has Pro had to make the 'abstain' option available? Is it worth it?
Elections cost massively, even in smaller nations.
In 2016, the US elections cost an estimated $6.5bn. BBC Reality Check breaks down who paid for it and looks at how much 2020 might cost.
This cost will undeniably go up in a compulsory voting scenario. There's far more votes to count (using the 'low turnout' point against Pro). You are essentially forcing people who were content not voting and wasting vote-counters' time to vote. Administration costs aside, the dynamics of campaigning will now revolve around subtley encouraging people to simply resent you less than your opponent rather than like you and root against your opponent.
The problem here is that some people who would have truly abstained from voting are not going to 'throw away' their vote nor are they going to vote accurately. They are going to vote for whichever Party grabbed their attention more, rather than the one they truly agree with. Why would they do this? People don't tend to like to throw away opportunities they have been forced to take.
This leads me onto my next section.
Fear as a motivator is toxic to Elections.
While Switzerland is an example of mandatory voting, they don't have abstention as a non-finable option and they don't regard it as disinterest:
So, I firstly want to disqualify it as a valid example of forcing people to vote in the context of this debate, where abstention is a valid option in the ballot paper that frees one from the fine that would be applied for refusing to vote.
Some people have jobs situations and/or mental health reasons to not feel free and/or fit to vote in an election. Some people don't want to deal with the anxiety and now you've forced them to 'fear their way into the voting booth'.
They will make their decision based on a psychology of being forced to be there, not one of wanting to, which will lead them to vote candidates who have either got authoritarian or libertarian ethos, not allowing middleground sensible ones. Why? Because a scared mindset isn't suitable to vote effectively in an election for people who hate pressure.