PR creates an unfair balance of power.  Coalition government is actually unfair, as small parties with only a few percent of support nationally can hold the balance of power. This can result in them being able to force through unpopular or sectarian policies with no national mandate as a price for their support in parliament; for example, the Dutch coalition lost its majority in 2011, meaning it may have to rely on the support of the SGP, a very small conservative Christian party that does not even allow women to be members (Financial Times, ‘Dutch Coalition loses Senate majority).  Particularly when there is only one potential small party that could be a coalition partner for the biggest party(s) that small party potentially holds a lot more power than their number of seats in parliament would imply. When either of the main parties could form a government the small party can negotiate with both to get the best deal possible. And once in government they can threaten to walk out if they do not get their way on the issues that matter to them.