I'm not going to quote anyone, but there's many things in this thread, that are just plain wrong.
Skating is not really physically harder. To be honest, for less then perfect trained athlete, classic is physically harder. If you can't go anymore uphill, with skating it still looks like you are moving somewhere, while with classic you are only walking. For racing, both are same... you are dead on the end of race, no matter if race is skating or classic
If you are not, you are doing something wrong
Next to technique... skating is smooth? Sure, but so is classic. But then again, if you look some beginner who thinks skating is easier it looks really bad. I have no idea why people think they can ski skating better then classic... better then movement in which they are moving for all their life (read: walking or running, just that here you added skis and poles). I guess it's partly because noone showed them movie of their skiing
For beginner classic is much easier, since movement is much more natural then it is with skating. To be honest, 99% of people I meet on skiing tracks nowadays have no idea how to properly ski skating. And the rest 1% are (ex) racers.
But there is one thing with classic, and that's waxing. It's no big deal if you are living up in Scandinavia and you have blue extra period since October till April, but for pretty much for all rest of World, waxing can be quite often pain in the a**.
Personally I wouldn't really go into road vs. mtb comparison with skating vs. classic. Maybe because when racing, you have to do both, so both skating and classic are for me, after 20 years of racing, just equal part of xc skiing. Xc skiing without one of them doesn't exist.