Leagues that I've been in that do those balance it out in a way that makes PPR and PPC about even. To keep the numbers simple, say that a good RB averages 20 carries and a good WR averages 10 catches, then they make the points something like 0.5 for a catch and 0.25 for a carry.
I've been in leagues that did it evenly... 1.0 per catch or rush, and it makes RB's way more valuable.