Remember all the cries of foul awhile back when GM revealed the Voltec transmission? At that time most folks believed that the Volt when operated in the range extender CS mode is a simple serial hybrid, i.e., ICE powers a generator, which makes electricity to turn the traction motor. As it turned out, the Voltec transmission had 4 modes, the last of which caused all the fuss. In this mode the ICE, M/G and ring gear were all connected together. The problem for the critics was GM called the Volt an EV, and now there was a direct connection between the ICE and the wheels, making it more Prius like, and thus the cries of "GM lied".
GM said this was done for efficiency. This paper models the Voltec against a serial hybrid and determines the relative efficiency of each approach.
I won't pretend to say I totally understand the contents as it tends to be quite technical, but the conclusion is clear; "It was determined that the Voltec powertrain achieved lower levels of fuel consumption during all driving condition modes, as compared with a pure series configuration."