My point is more, if you give a rogue saying "you do more damage to the target", that you should expect blade flurry to be used to copy that damage. Because that's how flurry works.
"Even as it is now if it's a buff on you that damage modifier applies to the cleaved target independently of the original so that it doesn't double dip."
Buffs on YOU make the damage get copied.
Debuffs on the target make the damage not get copied.
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I think his point is mostly- if you COULD use the damage correctly, if it could be done properly, that combat would be on top. By a lot. The fact that in practice this isn't even close to true mostly ends up meaning that the damage lost is quite intense.