Just FYI, the resistance adjustment on an erg isn't just a matter of easy versus hard. When you set the resistance higher, you are mimicking the requirements of rowing in a smaller boat. In a smaller boat (like a single) the boat slows down appreciably between strokes, so every stroke is "harder" because you have to pull harder against the water. It requires a bit more work in the arms and back to finish the stroke. At the lower resistances, the machine mimics the feeling of rowing in a full eight man boat, where the water is going by quickly and the boat doesn't slow down much between strokes. So you drop your oar in and explode through the legs. The arms and back don't do much but complete the motion. On a Concept2, that's around the 4-5 resistance mark, and it is the range where most people train.
Its not better to row on a harder resistance for two reasons: the first is that the computer automatically adjusts for your resistance anyway, so rowing on a lower setting doesn't show a faster speed. For that reason competitors at indoor rowing competitions are allowed to choose at which resistance they would like to compete. Secondly, someone without much experience or proper technique could easily put too much strain on their lower back by rowing at a unnecessarily "hard" resistance just because they think harder is better. If you're too weak to row quickly at that resistance, you're also going to be cheating yourself out of a good workout.
Just some info for anyone who happens to read this. There's nothing wrong with rowing at a higher resistance if you like the way it feels, as long as you're still exploding from the legs at the beginning of the stroke and properly bracing your core to avoid injury. I see a lot of guys jump on the machine and bump the resistance all the way up and then row with really ugly technique, without realizing they could be much better off working on technique and explosiveness at a lower resistance level.