You would have the option to not actually vote for anyone if you didn't know who to vote for. Compulsory voting only means you have to drag your butt down (or send a mail) and is therefor forced to make a conscious decision to vote or not. This way everyone that actually have an opinion will vote (They might as well now they are forced down there) and those that are clueless would just vote blank.
It will in the end give a better picture of what the entire country wants, and not only the views of those die hard political active, that wouldn't mind standing in ques for 5 hours.