Even if a poll of all eligible voters is more noisy or just worse than a poll of typical voters, compulsory voting changes the game from the point of view of the politicians. In the US, convincing people on your side to vote seems to be a lot more effective than convincing people to switch sides. Compulsory voting changes the relative value of the two strategies, perhaps making voter turnout irrelevant. I don't know if such a change would be good or bad, but it sounds big to me. I also don't know what campaigns in actually existing compulsory voting regimes look like.