Fair votes would remove the advantage given to the main parties

With pr there is rarely an instance of a single party gaining an outright majority, they must form a coalition in most cases, but with fptp it is possible (likely) that a single party will have overall control. Power (with fptp) is concentrated among the two main parties. Fair votes would level the playing field by removing this advantage, so pr removes this unfair advantage.

There would be no advantage for the main parties under pr.