The merits of PR rules are known from studying their use in many countries. Some people fail to see the benefits of ensemble councils and balanced policies. And some who favor elections by Condorcet's rule say they don't see any value in broad representation. But such rules lead to narrow and one-sided policies.
Balanced majorities avoid policy reversals and thus save money and maintain credibility.
They show the rule of reason not of whim, thus inspire confidence in legitimate leadership. They do not let fringe reps steer policies, a common fact in one-sided majorities. Popular belief that government exists for the general good not just for the strongest factions is hurt by one-sided policies. But balanced policies favor all moderates thus increase satisfaction and reduce political conflict. They have broad appeal and thus help the organization attract members.