Of course it's helpful, unless one has the ridiculous idea that democracies can't make mistakes.

Yes, voters are acting in their own interests, or at least what they perceive to be their interests at the time. This can lead to bad outcomes. It's why we have things like the Bill of Rights, which restricts what even a majority can impose.

The idea that voters are always right would also seem to eliminate the possibility of arguing for change. Voters are responsible for current policy therefore it must be right, so how could change be good?