Not true. Eco-cruise does in fact maintain a speed, it just does it with an eye to avoiding lead-footed acceleration. You set it on 60 and you're going 60 most of the time.
In response to the original post---you'll find that you'll be on electric about 60% of the time in the city.
Correct. Eco Cruise does work to maintain your speed at your selected set point but without jerking around your throttle when hills, valleys, headwinds and tailwinds change.
So, here's a little chestnut of experience from Prius-land which might paint the difference between Eco Cruise and standard cruise. In my Prius, if you hit an incline while using cruise the vehicle will throttle up as soon as it notices an incline and senses reduced speed. The Prius's cruise is so sensitive that when you hit a hill and you are doing highway speed (70mph in my neck of the woods), the vehicle will throttle up, sometimes quite noticeably, to compensate for the hill. The Prius is the only vehicle I've ever driven which can hold a cruise set-point without loosing speed. When you set it at 70, it stays at 70. It is remarkable. The Prius does not have an Eco Cruise equivelant.
I've never turned Eco Cruise off on the C-Max, so I'm not sure what it's behavior would be in changing topology, but I suspect that with Eco Cruise off, the vehicle would behave much the same, speed control behavior as the Prius, using aggressive throttle response to maintain the set point. Eco Cruise reduces the aggressiveness of the throttle response, allowing the vehicle to drift down to a slightly slower speed when hills are encountered and then more gradually coming back to the set point without goosing the throttle.