There may be many unforeseen consequences with such a change:

Do you allow normal comments in response to fluff comments? 1) Yes - you'll need to reenable fluff to follow what the "serious" replies are referring to. 2) No - the responding comments would by necessity also be labelled fluff, even if intended seriously (just to enable being able to follow up on some throw-away remark), which would lead to a continuing incentive for fluff comments - actual fluff and otherwise - to supplant actual comments. 3) No replies allowed to fluff comments - weird circumvention effects e.g. by replying to the grandparent while referring to the fluff comment

Don't you want to read all the content some specific authors provide on LW, regardless of whether they consider it serious or just "candy-floss"? Do you then have an option for "hide fluff - except for fluff from users 1..n"? Yes - keeping such a list up to date could quickly become tedious. No - have to reenable all fluff unless you want to miss e.g. Nesov's fluff-labelled comments, which will probably still be more valuable than most people's serious comments.

The list goes on. (Eventually some may even wish for a way of rating fluff comments, leading to a secondary karma system, which may lead to a regress until someone pulls the plug and goes back to the current system.)