Idk, the people I listed are the strongest and they didn't think it was necessary while they had access to it.
Sure.
That doesn't contradict my point though. Even if buff provided by DF is say 10%, that is still a significant enough buff.
And when it comes to the most powerful DFs, that isn't even the case. I mean the buff is substantial.
With Marco's DF, Roger wouldn't have to worry about taking damage.
Base Luffy is at minimum a tier below his G4/G5.
Logia DFs in the hands of Roger, Mihawk, Shanks etc would at least jump them up a sub tier, if not a tier. Even if the DFs can't provide attack power comparable to advanced Conqueror's swordsmanship, the boost to CQC from the Logia would itself make a significant difference. At their levels of speed and observation haki, they would be doing Katakuri stuff. Same for any mythical zoan, just keep being swordsmen but with heightened speed and strength in hybrid.
Etc.
There is no practical reason they don't eat a good DF