If we had proportional representation, we would have a lot of little parties all of which would be concerned with ideological purity (in order to maintain their electoral base) and patronage (in order to keep a cadre of party workers). The French Fourth Republic and post-WWII Italy seem odd to adopt as models.
I think our system--two broad coalitions, each with its high politicians desperately lunging for the middle for Downsian reasons--is broadly superior because it tends to favor politicians who are good at assembling coalitions as opposed to those who are best at pleasing their base.
But the big issue is not PR vs. FPTP, but how the electorate is constituted...
Brad DeLong