There are a number of benefits:

1. InnoDB has certain optimizations that PG lacks which can make a big performance difference at the high end: index-only queries, insert buffer (or change buffer in MySQL 5.5+), clustered index

2. Lightweight connection creation: MySQL can handle many more concurrent connections and also can create new connections much faster due to threading vs. process model

3. More flexible replication: PG is catching up, but MySQL still has the edge here imo.