The difference is that the Volt's ICE was also tied into the driveline, so it wasn't just a generator, which means it had to work harder at highway speeds, not just spend more time running.
Not sure if this is a real thing or not but this little generator in a trailer captures the idea of an EREV in my mind. I get ICE, HEV, PHEV and BEVs but not so much for the EREV.
Let's see. Ford will soon offer buyers the choice between ICE's, conventional hybrids, PHEV's, EREV's and full EV's. Something for everyone. What a novel idea, give the customer a choice and let them decide.
The 2009 was all-new except for the powertrains and the visual appearance of the cab and doors--structurally, the cab was different, to meet new rollover standards. That's why they no longer had the B-pillar windows in the '09s (and newer) regular cabs.
In the case of the Ford Escape the HEV is 2 MPG better on the highway than the ICE (36 vs 34). The EPA does not report the highway MPG of the PHEV but I think I seem to remember it being one less that the HEV making it 35. For this particular comparison the hybrids are around 3-5% more efficient on the highway (not very significant). As mentioned above, the EREV has the disadvantage of converting mechanical energy to electrical and then back to mechanical again. That may outweigh the advantage of generator constant speed operation as it did for the Volt.
Given those parameters, ICE would very likely be on top. For the Fords I compared, there was very little to no difference between the EPA highway ratings for the Hybrid and ICE models. BEVs would be last by a country mile due to the inconvenience and peace of mind factors. EREVs could be at or near the top, but it depends on how they're executed--if they're done like @fordmantpw suggested and designed to keep the engine in its most efficient range, they should lose less efficiency than HEVs or PHEVs at highway speed because they'll just be running more, not working harder, considering that their ICEs are just running as generators, not providing propulsive power.
An EREV adds yet another power train choice for consumers to choose from. Having multiple options is good but it is getting more complicated for the non-enthusiast.
For the case of highway driving on longer trips (e.g. family vacation, snowbird destination travel etc) and given the current state of charging infrastructure and battery technology I would rate the different power train options in this order (best to worst) based on convenience and peace of mind:
HEV
PHEV
ICE
BEV
EREV
Am I close or way out in left field? The issue becomes a lot more complicated with many more variable for city/local driving.