There are charts somewhere on the web that show the efficiency losses of the Rohloffs, compared with derailleurs, for each gear.
For racing, it could be a very legitimate concern. But in other cases -- when you aren't very pressed for time, or trying to keep up with fast riders, or in some other situation where two or three or four percent losses might matter -- it may or may not be significant.
One percent seems pretty minor. Two percent also seems minor. But when you get up around three or four percent, it starts to make one wonder.
Then again, maybe it doesn't really matter.
If anyone has any viewpoints or perspectives on this, please feel free to post them.