I imagine it all goes to purpose. Some may find them useful for a large chopper/camp knife to help with retention. Hunting knives--
I see no real purpose for it there. Small edc's are benefited by the assist they give in removing a small knife from a whole- knife, form-fitted sheath. When only an inch or so of the handle butt is protruding from the sheath, your pinky and ring finger naturally encircles the lanyard/fob giving you positive purchase on the knife, and the knife is naturally indexed into your palm securely. No struggle to draw the knife, no fear of dropping the knife.
Dave Fairly uses a leather lanyard on his western/cowboy edc's, and has posted pics of real cowboys using them during branding/cutting time. If I recall correctly, the lanyard aids with knife retention when the hand(s) are slick with blood. Dirty lanyards are easily replaced.
But, it is a personal choice I guess as, I sell a good number of my edc's with the lanyard and Dave sells a gob of them.
In the end, it comes down to personal preference.