Fptp at least makes the government look bad

First past the post is conservative in that it fails to give either of the main parties a true mandate to enact their policies. If there is proportional representation and a party wins a convincing majority there is no thought that they have anything other than a true mandate. If there is first past the post there exists a sense that things might have been different under a true voting system. The main aim under fptp is to exclude the opposition which is more conservative than to gain power for its own sake. It is more convincing to win a majority under pr since there was no tactical reason to vote for your party. If you get a majority under pr then it is a true reflection of people's wishes but a majority under fptp has the understanding that it would not have been achieved without fptp. Fptp introduces an element of doubt as to the authenticity of the win.