One thing I've noticed is not inefficiency, but it seems the drive train inherently gets more awkward. An upright bike consists of a bike made for maximum simplicity, and then the person contorts himself to fit. The recumbents start with the person and contort the bike around them.
The issue of pushing hard against the pedals I suspect is a non-issue. I ride a single-speed upright bike. But I don't know if I've ever had to stand on the pedals. The problem isn't needing to give a mighty heave-ho one time, the problem is having to put 30% of your weight on that pedal over and over and over. If you were having a hill-climb on a 10' course, then there might be an advantage to having something to brace your butt against while you pedaled.
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"be careful this rando stuff is addictive and dan's the 'pusher'."