There is no reason to abstain in Australia because we have a preference system. In the States, when Nader ran against Gore it divided the Democrat vote and Bush won. The same thing doesn't happen in Australia so if you vote for a minor party because you really believe in their policies, although they may still preference a major party that you don't wholly agree with they still get funding based on the number of votes received and if they actually win a seat in parliament (as Adam Bandt did in his recent historic win in the federal seat of Melbourne for the Greens) then those "minor" parties start to have a "major" influence.
All in all I think the preference system and mandatory voting makes for a far healthier democracy.
All in all I think the preference system and mandatory voting makes for a far healthier democracy.