Another factor that PR proponents should consider, is what limits, if any, to put on the degree of proportionality supported. As a cautionary tale, let me cite this description of the doomed Weimar republic:
“… the Weimar Constitution had serious problems.
“The use of an excessively proportional electoral system was one such problem. This system, intended to avoid the wasting of votes, allowed the rise of a multitude of splinter parties, many of which represented the extreme ends of the political spectrum. This in turn made it difficult for any party to establish and maintain a workable parliamentary majority.
“This factionalism was one contributing factor in the frequent changes in government. Shirer cites the presence of some 28 political parties in the 1930 national elections; Otto Friedrich cites 40 different groups in the Reichstag in 1933. There was no threshold to win representation in the Reichstag, and hence no safeguard against a quick rise of an extremist party.
“It was possible to win a seat in the chamber with as little as 0.4 percent of the vote. In the 1924 elections where[sic] the Bavarian Peasants’ League got just 0.7% of the vote, enough for three seats in the Reichstag.” — Weimar Constitution, Weaknesses, Wikipedia
So, yes, there can indeed be too much proportionality. It has been said, reprised, and paraphrased many times over the years, that “Those who don’t know history are destined to repeat it.” — Edmond Burke (1729–1797)
This is not by any means to suggest that PR necessarily leads to the same demise as befell the Weimar republic, just that we want to ensure suitable bounds to it, such as a requirement for some minimum level of support, say, 5%, in order to allocate proportional seats.