I am WELL confused! Bear with me a moment. There is a question in all this somewhere!
XC skis are curved (camber?). The heel and toe sections of the ski are always in contact with the ground and provide the glide, the central section only comes down on the snow when sufficient force is appplied.
The "springiness" of the camber of your ski has to be strong enough to support your weight and keep the grip section off the snow if you're going to glide much. Weight is actually a force, and there seem to me to be FOUR basic weight/force conditions in classic skiing: 1: weight equally balanced between both skis; 2: weight on one ski but with the other resting on the ground; 3: with all your weight on one ski; 4: pushing off / kicking (on one ski). Condition 1 applies the least down-force on the ski, condition 2 more, condition 3 more still, and condition 4 the maximum downward force.
The right length of ski is determined by the person's height (assuming you're using ski tracks), so, if all the above is true, there are only two things needed to select the right ski: Your weight, and you fitness level. That's it.
The weight aspect is I think as I've described above. The fitness aspect is relevant because if your ski is "hard" sprung, you'll have to "time" your kick more precisely and kick harder to get the ski to contact the ground and grip, than with a "soft" sprung ski. But you'll also have less or no drag during condition 3, and therefore longer glide and more efficient use of your hard work. If in condition 3 the grip area of your skis is already touching the ground then you're already beginning to be slowed down and all that effort is being drained away.
So, basically the questions it seems to me that need to be answered in order to select a ski are: How much do you weigh? and: How fit/strong are you?
That's it. So what is all the hype about skis, other than trendy flashness and the creation of sales volume for the manufacturers?? What have I got wrong here? What have I missed?