Well, it may be a bit over-rated, but there are a few things that are agredd upon:
1) Every extra gram of rotating mass affects acceleration.
2) Every extra gram of total mass required more work to move and to raise up (as in climbing a hill).
3) Every extra gram of mass on a bike is extra unsprung mass, which has a negative effect on the ride of the bike.
Everything else is just a matter of degree. You want to be fast/smooth? How much are you prepared to spend?
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