The beauty of a democracy is that the people have a voice in their government. The voice might be loud--as in full participation in the process. The voice may be nominal--as in voting, though perhaps without much interest or passion. The voice may be silent--as in choosing not to speak at all. The ugliness of democracy is that often it's those who participate least who expect the most from their government. Forcing them to vote would ultimately cost everyone else.
I think that the issue has been addressed at length here. I would only like to expand on a potential con I see with it to which I had alluded in a previous answer. I see a fundamental tension with a constitutional democracy that is predicated upon individual rights and mandating that a person votes. When we examine the growth and evolution of rights based political systems, I think that it becomes clear that there was a distinct movement from a political order that stressed rights, but wished for everyone to embrace the political notion of the good to one that stressed rights and the pursuit of any expression of it. In Classical times, the use of freedom to advance political articulations of it was critical in asserting one's own state of liberty. Individuals who lacked political freedom were slaves and others that lacked rights. Therefore, Classical social orders were driven to ensure that freedom was there for those who could have it, and that these individuals pursue political participation as a way of enhancing their own autonomy, or state of being free. The modern conception of rights based government adopted the Classically Liberal idea that the notion of self- interest, not political autonomy, should guide one's pursuit of freedom. This shift was helped in the advance of commerce and business as ends that individuals could pursue. No longer were those who pursued monetary based ends seen "lesser" beings, but rather as active and vital participants to a social and political order. The result of this was greater freedom for all, but also a potential devaluing of the political notion of the good, helping to decrease political participation. It was a natural consequence, though, because a pure embrace of individual rights would mean that any path can be pursued and all are seen as equally valid and important, so long as individual freedom is being acted upon and acknowledged. The freedom to choose a life of baking cookies, cleaning vintage cars, running for political office, or counting blades of grass are all embraced in this model. With all of this in mind, if an individual chooses to not do something, then that, too, is an expression of their freedom and individual will. If they choose to not engage in business or social callings, rights based governments have to live with this reality. This extends to the issue of voting. Hence, the dilemma becomes that if a nation is committed to individual rights and the choices that extends from them, it seems to represent a slight contradiction to be able to say that those choices "have" to include the political expression of the good. This would be my problem with mandating voting under penalty.
The typical arguments for having compulsory voting are that voting is a civic duty and that it is important to have the voices of all people heard.
In a democracy, people say, it is important for all people to vote so that the vote will accurately reflect the desires of the whole population. Therefore, it is good to have voting be mandatory so that no one's voice will be left unheard.
It is also said that citizens in a democracy should vote because that is what makes a democracy. If you people do not vote, they become disengaged and no longer care about their country. By requiring them to vote, you make them more likely to be engaged in the civic life of the country. This makes your country stronger in terms of having a population that is involved and interested in its public affairs.
Purpose and the rationale for election of public servants by voting is that the people will elect only those candidates who are capable and work to promote best interests of the people. However the reality does not always work this way. One of the shortcoming of the election system is that not all people eligible for voting take the trouble of doing so.
Some people have suggested a system of compulsory voting by all people to overcome this problem. The people who support this approach argue that this will ensure that the people winning the election truly represent the choice of the majority. This is the only argument favouring compulsory voting. But in reality, forced vote is less likely to reflect a meaningful choice as compared to voluntary vote.
To be able to vote meaningfully and wisely, a voter must make fairly well informed about various issues facing the people, and the ability and sincerity of the candidates to address these issues honestly and effectively. This requires considerable effort. As it is, many people who vote, do so with inadequate understanding of the worthiness of various candidates. They vote based on inadequate information and analysis. Which results in voting for power to candidates who are not really the best persons for the job. Compulsory voting may force the persons to physical cast their vote, but it does not ensure that they will put in enough efforts to evaluate merits of different candidates and make correct voting choice. The people who do not vote without compulsion are the ones who take less trouble to evaluate the candidates. Compulsory voting is not likely to make them put in greater efforts for this. Thus their votes are likely to be more arbitrary and less reasoned. In this way the results of voting with compulsory voting is likely to be less reliable than voluntary voting. It is better to go by votes of fewer people who vote on the basis of better information and analysis, rather than to go by votes of larger number of people who vote arbitrarily.