What additional expressivity does polyvariance give in pushdown CFA?
After playing with an implementation (found here), I believe I've figured it out.

Although 0PDCFA technically entirely eliminates return-flow merging, this isn't a very strong result in the absence of any polyvariance: if the same function is called in two different contexts, the analysis must end up merging flows within the body of the function, since multiple values are bound to the same variable and the variable's abstract address is identified with its name. 1PDCFA provides a different set of addresses to each syntactically distinct call of each function (I might be either under- or over-selling here), eliminating much of the merging.