On one engine, I can do 25 knots for about 30 L/hr...
its the fact I have more drag in the water, more weight at back of the boat... etc...
The extra cost in fuel is far outweighed by the safety of having two completely seperate systems (fuel tank, battery, engine)... if either engine/fuel tank/battery fails, I still have 25 knots in her to get me home.
A lot of people have small auxilleries on there 6.5m+ boats, which would struggle to push them against the tide/wind... I quite like the fact that, say if mid channel the engine failed for whatever reason, I could still get back without the need for a tow