well of course any sacrifice in weight will help the rotational intertia and keeping the crank going, but without higher level material that runs a risk of sacrificing rigidity, which will help deliver more pedaling force through the chain rather than being used to bend the cranks.
higher rigidity often means decreased impact and stress strength however, because the lack of flex causes the 'breaking point' of the material to be lower... of course expensive, exotic materials can give you a better trade off, but all materials have certain physical properties... so you're not going to get the strength of steel with the weight savings of carbon... carbon is extremely rigid but not very strong for impacts...
basically it's a very complex set of physical properties that must balance off eachother.
the cassettes on the other hand are like gears in a car, if you want to get up a mountain(overcome gravity) you'll need a high gear when matched to the power the engine can put out... in a top speed situation you'll want a very low gear ratio so that one pedal will get you going a great distance.
for MTB riding it really all depends on your strength, and rigidity may not be so important as you'll want strength instead.