One reason to make voting compulsory in Australia was to make it difficult for employers to restrain their employees from voting by rostering them to work on polling day.
Personally I am strongly in favour of enforcing attendance at the polls; once a voter has had his name ticked off, and been issued a ballot paper, it is then up to him to decide whether to cast a vote or not; but that decision is not being made on the basis of 'Would I rather sit on my arse in front of the TV' - it is a decision not to vote, not a decision not to show up.
The low number of informal votes in Australian elections suggests that, once at a polling booth, most citizens do choose to cast a valid ballot. Few people genuinely have no preference at all; they may hate all the candidates, but there is generally one they hate less than the rest; and a system that forces them to express that preference is, IMO, more likely to produce a democratic outcome than one that allows the apathetic to casually discard the franchise.