I think it is a legitimate complaint that in proportional representation systems, where you have several parties and need a coalition of two or more parties to form a government, in the negotiations the different parties have to compromise and may not be able to stick to the promises they've made in their platforms. I think the people who negotiate are elected by the people. They try to be as faithful as possible to the promises they've made, but as minority parties, they are of course aware that they may not be able to have their way entirely.
What should also be pointed out is that in mature multi-party systems—and I'm thinking of, for instance, Germany—it is often clear prior to the election which parties are going to work together in a government. In the last election, there was clarity for the voters, that if they voted for one party, they were in fact voting for its coalition with other parties.