It forces parties to cooperate. It works brilliantly in many European countries. If UKIP got the 3rd largest portion of the vote then they should have got the 3rd largest amount of seats.
A Conservative - UKIP coalition would have filled me with with revulsion but it would have been what the electorate voted for. Of course you’re assuming that people would have voted in the exact same way under PR which is something you cannot begin to predict.