I think what some people (including me) are trying to say is...roughly this...oh and lets forget about wind resistance.
In theory..two bikes...one geared, one singlespeed. Riders of equal fitness etc etc...
Lets say the geared bike is travelling at 15mph. Lets say that equates to a comfortable cadence for the SS bike.
Go faster and the geared bike simply changes gear...with a bit more effort. I would say the SSer would have to put in significantly more effort to maintain that speed.
The same for going up hill etc
Extrapolate that over a ride, and the SSer is putting in more effort for the same results plus a greater variance in cadence = using different muscle groups...legs and upper body.
But at the end of the day....who cares
I'm definately fitter for riding SS....and thats all that counts to me