Without pr there is no representation

The state must be representative which means the people must be able to vote for a candidate with whom they agree. If there is a fully-proportional system in place then the people will be able to vote for someone who represents their beliefs and interests. This means that they have no reason to consider the likely actions of the rest of the electorate (to vote tactically). If tactical voting is rewarded then it is almost certain that a proportional system is not in use. If there is a system where only a limited number of candidates are to be advanced from each seat as with first-past-the-post then people will be rewarded for voting tactically and the system is not proportional. If there are not enough winners in each seat (with the extreme case being only one) then the voting system has been corrupted and the authority of the state is compromised. To be valid the state should allow more than one person to advance from each seat with the higher number being the most democratic. In the extreme case of (direct) democracy we have a referendum where effectively everyone is a parliamentarian and the number of MPs is equal to the population. If the number of MPs advanced from each seat is sufficient then voters will have no reason to be tactical in their choices and the parliament can be said to be fully representative.