It would be more accurate to say that PR gives more appropriate influence to third parties. It's true to say that political power/influence is not generally pr

It would be more accurate to say that PR gives more appropriate influence to third parties. It's true to say that political power/influence is not generally proportional to parliamentary party size, but is not specific to PR. For example, First-Past-The-Post typically gives disproportionate influence to the first and second parties (i.e. 100% of the power), which greatly reduces voter ability to express their preferences as politicians are forced to coalesce on one side or another of an arbitrary line. PR typically benefits parties who are able to negotiate and find common ground with their opponents. An extremist party will (still?) be left out in the cold. Finally: Arrow's Theorem isn't relevant to PR; it discusses transferable/ranked voting systems.