If the voting people don't hold politicians accountable for their election campaign promises then the whole idea of democracy put in practice is flawed.
It makes politicians use almost all means possible to win an election. Lying, manipulation, vote buying, even violence if desperately needed. It degrades elections to mere popularity contests. You know, like the ones you had in school?
And do you really want to give the decision power regarding, for example, the military force of a country to a person who wants to win a popularity contest?
So, the question is, what is one really going to do if their elected politician goes against their campaign promises? If the answer is resignation, rinse and repeat then the voting populace is either lazy or uneducated. Voting is only half of it's political responsibility. Holding politicians accountable is the other half.